This is the auth flow for valorant (async) + Some further research I did in the past

auth_flow.py

import re
import aiohttp

async def run(username, password):
    session = aiohttp.ClientSession()
    data = {
        'client_id': 'play-valorant-web-prod',
        'nonce': '1',
        'redirect_uri': 'https://beta.playvalorant.com/opt_in',
        'response_type': 'token id_token',
    }
    await session.post('https://auth.riotgames.com/api/v1/authorization', json=data)

    data = {
        'type': 'auth',
        'username': username,
        'password': password
    }
    async with session.put('https://auth.riotgames.com/api/v1/authorization', json=data) as r:
        data = await r.json()
    pattern = re.compile('access_token=((?:[a-zA-Z]|\d|\.|-|_)*).*id_token=((?:[a-zA-Z]|\d|\.|-|_)*).*expires_in=(\d*)')
    data = pattern.findall(data['response']['parameters']['uri'])[0]
    access_token = data[0]
    print('Access Token: ' + access_token)
    id_token = data[1]
    expires_in = data[2]

    headers = {
        'Authorization': f'Bearer {access_token}',
    }
    async with session.post('https://entitlements.auth.riotgames.com/api/token/v1', headers=headers, json={}) as r:
        data = await r.json()
    entitlements_token = data['entitlements_token']
    print('Entitlements Token: ' + entitlements_token)

    async with session.post('https://auth.riotgames.com/userinfo', headers=headers, json={}) as r:
        data = await r.json()
    user_id = data['sub']
    print('User ID: ' + user_id)
    headers['X-Riot-Entitlements-JWT'] = entitlements_token
    
    # Example Request. (Access Token and Entitlements Token needs to be included!)
    async with session.get(f'https://pd.eu.a.pvp.net/match-history/v1/history/{user_id}?startIndex=0&endIndex=10', headers=headers) as r:
        data = json.loads(await r.text())
    print(data)

    await session.close()

if __name__ == '__main__':
    asyncio.get_event_loop().run_until_complete(run('exmaple user name', 'my_secret_password'))

endpoints.txt

POST {URL}/rso-auth/v2/authorizations

POST {URL}/rso-auth/v1/session/credentials
Payload: {"username":"{USERNAME}","password":"{PASSOWORD}","persistLogin":false}
Response: {"country":"deu","error":"","multifactor":{"email":"","method":"method_not_set"},"persistLogin":false,"type":"authenticated"}

GET {URL}/riot-client-auth/v1/userinfo
Response: {"acct":{"game_name":"{RIOT_USER_NAME}","tag_line":"{RIOT_USER_TAG}"},"country":"deu","email_verified":true,"lol_account":{"summoner_name":"Luc1412"},"phone_number_verified":false,"player_plocale":null}

GET {URL}/rso-auth/v1/authorization
Response: {"currentAccountId": {ACCOUNT_ID},"currentPlatformId":"EUW1","subject":"b639ec2f-e2a0-509a-85f2-ffbec2526938"}

GET {URL}/rnet-product-registry/v4/products
Response: [{"id":"valorant","name":"VALORANT","patchlines":[{"auto_patching_enabled_by_player":false,"configType":"player","content_paths":{"loc":"https://valorant.secure.dyn.riotcdn.net/channels/public/rccontent/loc","riotstatus":"https://valorant.secure.dyn.riotcdn.net/channels/public","social":"https://valorant.secure.dyn.riotcdn.net/channels/public/rccontent/social"},"default_theme_manifest":"","dependencies":[{"args":[],"elevate":false,"hash":"","id":"vanguard","min_version":"","url":"","valid_exit_codes":"","version":""},{"args":["/quiet","/norestart"],"elevate":true,"hash":"","id":"VC++","min_version":"14.0.23026","url":"https://valorant.secure.dyn.riotcdn.net/x/riotclient/dependencies/vc_redist_x64.exe","valid_exit_codes":"1638","version":"14.0.23026"},{"args":["/silent"],"elevate":true,"hash":"","id":"DirectX","min_version":"1.0.0","url":"https://valorant.secure.dyn.riotcdn.net/x/riotclient/dependencies/DirectX_20190310.exe","valid_exit_codes":"","version":"1.0.0"}],"dependencyStates":{"DirectX":{"hash":"7eaf6eb2cfefebf2be59e43598e2bc881eb9012285a7bc2eedbb3d732466919b","phase":"Succeeded","version":"1.0.0"},"VC++":{"hash":"b6cc20d1e0b59ef7e485c2c2a8dfebd7f3ebc1674a21dbc146adc8d84e88d41c","phase":"Succeeded","version":"14.0.23026"},"vanguard":{"hash":"","phase":"NotInstalled","version":""}},"full_name":"VALORANT","id":"live","install_dir":"VALORANT/live","install_full_path":"D:/Program Files/Riot Games/VALORANT/live","install_id":"valorant.live","install_size_bytes":0,"launcher_arguments":[],"locale_data":{"available_locales":["en_US","de_DE","es_ES","fr_FR","it_IT","ko_KR","pl_PL","ru_RU","tr_TR"],"default_locale":"en_US"},"patch_notes":"","patch_status_known":false,"patching_ask_policy":"ask","patching_policy":"manual","path_name":"VALORANT","platform":"win","primary_executable":"","region_data":null,"release_id":"198A6AB4AADCDCB3","rogue_process_whitelist":[],"root_dir":"D:/Program Files/Riot Games","secondary_patchlines":[],"settings":{"create_shortcut":false,"create_uninstall_key":true,"locale":"en_US"},"shards_data":{},"theme_manifest":"https://valorant.secure.dyn.riotcdn.net/channels/public/rccontent/theme/closedbeta/default/manifest.json"}]}]

GET {URL}/rso-auth/v1/authorization/access-token
{"expiry":1587164699,"scopes":["openid","link","lol_region","lol","summoner","offline_access"],"token":"{TOKEN}"}

GET {URL}/entitlements/v1/token
{"accessToken":"{TOKEN}","entitlements":["urn:entitlement:valorantriot.valorant.closedbeta"],"issuer":"https://entitlements.auth.riotgames.com","subject":"b639ec2f-e2a0-509a-85f2-ffbec2526938","token":"{TOKEN}"}

GET https://auth.riotgames.com/userinfo
Headers: Authorization: Barear {URL}/rso-auth/v1/authorization/access-token

GET https://shared.eu.a.pvp.net/v1/config/eu
Headers:    Authorization:              Barear {URL}/rso-auth/v1/authorization/access-token
            X-Riot-Entitlements-JWT:    Token from {URL}/entitlements/v1/token


INFO ABOUT  COMPETETIVE Tiers, last rank changes?
GET https://pd.eu.a.pvp.net/mmr/v1/players/{USER_ID}

Get data about the current players ingame profile settings (Background Card, Weapon Skins etc.) Maybe combine these with data drom datafiles
https://pd.eu.a.pvp.net/personalization/v2/players/{PLAYER_ID}/playerloadout

Get info about players contracts
GET https://pd.eu.a.pvp.net/contracts/v1/contracts/{PLAYER_ID}
Auth + Riot entitlements

Get current party
GET https://glz-eu-1.eu.a.pvp.net/parties/v1/players/{PLAYER_ID}?aresriot.aws-rclusterprod-euc1-1.eu-gp-frankfurt-1=50&aresriot.aws-rclusterprod-eun1-1.eu-gp-stockholm-1=31&aresriot.aws-rclusterprod-euw3-1.eu-gp-paris-1=58&aresriot.mtl-riot-ist1-2.eu-gp-istanbul-1=81
Auth + Riot entitlements

Get matches (max. 20 at once)
GET https://pd.eu.a.pvp.net/match-history/v1/history/{PLAYER_ID}?startIndex=0&endIndex=10
Auth + Riot entitlements

Get Match Info:
GET https://pd.eu.a.pvp.net/match-details/v1/matches/ec9a43b8-3a94-44e4-83c2-daf31a0b0bed
Auth + Riot entitlements

Store Stuff:
https://pd.eu.a.pvp.net/store/v1/offers/
https://pd.eu.a.pvp.net/store/v2/storefront/b639ec2f-e2a0-509a-85f2-ffbec2526938

Is it possible to get the Entitlement token with official Riot OAuth?

Hey I tried executing your script but I doesn't seem to work.
I'm getting errors specifically at Line 19/20

    async with session.put('https://auth.riotgames.com/api/v1/authorization', json=data) as r:
        data = await r.json()

Is it possible to get the Entitlement token with official Riot OAuth?

Yeah

headers
{
            'Content-Type': 'application/json',
            'Authorization': f'Bearer <access_token>'
}
url
https://entitlements.auth.riotgames.com/api/token/v1

better later than never lol