Check discord names using the pomelo-attempt endpoint. This endpoint is only available if you have the option to pick a new name for the first time.

discord-name-check.js

async function processNames() {
  for (const name of namelist) {
    try {
      const res = await makeRequest(name);
      
      if (res.ok) {
        const data = await res.json();
        if (data.taken) {
            console.log(`[-] ${name} was taken.`)
            continue;
        }
         console.log(`[+] ${name} is available.`)
        });
      }
    } catch (error) {
      console.error(`An error occurred while processing username ${name}:`, error);
    }
  }
}

async function makeRequest(name) {
  const res = await fetch("https://discord.com/api/v9/users/@me/pomelo-attempt", {
    "credentials": "include",
    "headers": {
        "User-Agent": "",
        "Accept": "*/*",
        "Accept-Language": "",
        "Content-Type": "",
        "Authorization": "",
        "X-Super-Properties": "",
        "X-Discord-Locale": "",
        "X-Discord-Timezone": "",
        "X-Debug-Options": "",
        "Alt-Used": "",
        "Sec-Fetch-Dest": "",
        "Sec-Fetch-Mode": "",
        "Sec-Fetch-Site": "",
        "Sec-GPC": ""
    },
    "referrer": "https://discord.com/channels/@me",
    "body": `{\"username\":\"${name}\"}`,
    "method": "POST",
    "mode": "cors"
});
  
  if (res.ok) {
    return res;
  } else if (res.status === 429) {
    const retryAfter = res.headers.get('Retry-After');
    const delay = retryAfter ? parseInt(retryAfter) * 1000 : 1000; // default delay of 1 second
    
    //console.log(`Rate limit exceeded. Retrying after ${delay / 1000} seconds.`);
    await sleep(delay); // wait for the specified delay before retrying
    return makeRequest(name); // retry the request
  } else if (res.status === 401) {
    throw new Error(`You do not have access to this endpoint.`);
  } else {
    throw new Error(`Request failed with status ${res.status}`);
  }
}

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

processNames();

How to get proper fetch data:

1. Open Developer Tools when picking a new name for the first time.
2. Go to Network Tab
3. Try to pick a name that is taken (e.g. "xd")
4. There should be a 400 status request after doing the above.
5. Right click and click on "Use as fetch in console"
6. Paste into script, but make sure that lines 40-43 are the same

Doesn't really work anymore, since Discord is heavily rate limiting the pomelo-attempt endpoint.