This is a Python script to delete Slack unnecessary messages. When the suffix of the channel name is "_bot", deletes the message that has passed 48 hours or more.

requirements.txt

requests
datetime
timedelta
python-dotenv

slack-gc.py

# encoding: utf-8

# _で始まるチャンネル       →削除しない
# 普通のチャンネル          →1ヶ月で削除
# _botで終わるチャンネル    →48時間で削除

import os
import sys
from os.path import join, dirname
from dotenv import load_dotenv
import requests
import json
from time import sleep
from datetime import datetime, timedelta
import re

SLACK_ACCESS_TOKEN = ""
LOG_CHANNEL_NAME = ""
LOG_CHANNEL_ID = ""

def delete_channel_message(ch_dict, limit_date):

    global SLACK_ACCESS_TOKEN

    log_message = ""
    #各botチャンネルで48時間以上経過した投稿を削除する
    for del_ch in ch_dict:
        del_ch_id = del_ch[0]
        del_ch_name = del_ch[1]
        del_message_count = 0

        #チャンネルから1000件のメッセージを取得(これが最大値)
        res = requests.get('https://slack.com/api/channels.history?token=' + SLACK_ACCESS_TOKEN + '&channel=' + del_ch_id + '&count=' + '1000')
        channel_messages = json.loads(res.text)
        log_message = log_message + "\"" + del_ch_name + "\" has " + str(len(channel_messages['messages'])) + " msg."
        print("\"" + del_ch_name + "\" has " + str(len(channel_messages['messages'])) + " messages")

        # 各メッセージについて経過時間を確認
        now = datetime.now()
        for ch_message in channel_messages['messages']:
            message_timestamp = datetime.fromtimestamp(float(ch_message['ts']))
            #2日以上経過した投稿だった場合削除する
            if (now - message_timestamp) > timedelta(days=limit_date):
                del_message_count = del_message_count + 1
                requests.post('https://slack.com/api/chat.delete?token=' + SLACK_ACCESS_TOKEN + '&channel=' + del_ch_id + '&ts=' + ch_message['ts'])
                sleep(1)
        log_message = log_message + " " + str(del_message_count) + " messages has been deleted\n"
    return log_message


def main():
    global SLACK_ACCESS_TOKEN
    global LOG_CHANNEL_NAME
    global LOG_CHANNEL_ID

    dotenv_path = join(dirname(__file__), '.env')
    load_dotenv(dotenv_path)
    SLACK_ACCESS_TOKEN = os.environ.get("SLACK_ACCESS_TOKEN")
    LOG_CHANNEL_NAME = os.environ.get("SLACK_LOG_CHANNEL_NAME")

    log_message = "---\n"
    ch_dict = {}

    print("start at " + str(datetime.now()))

    # Slackからチャンネル一覧を取得
    res = requests.get('https://slack.com/api/channels.list?token=' + SLACK_ACCESS_TOKEN)
    channels_list = json.loads(res.text)

    # チャンネルIDとチャンネル名の辞書を生成
    for channel in channels_list['channels']:
        ch_dict[channel['id']] = channel['name']

    # suffixが"_bot"となっているチャンネルを取得
    bot_channels = [ch for ch in ch_dict.items() if re.match(r".*_bot$", ch[1])]

    # prefixが"_"となっているチャンネルを取得
    unimportant_channels = [ch for ch in ch_dict.items() if not (re.match(r"^_", ch[1]) or re.match(r"^general$", ch[1]) or re.match(r".*_bot$", ch[1]) )]

    # log出力先のチャンネルを特定(存在しないチャンネル名だとエラーになる)
    log_channel_name = LOG_CHANNEL_NAME
    log_channel = [ch for ch in ch_dict.items() if re.match(r"^%s$" % log_channel_name, ch[1])]
    LOG_CHANNEL_ID = log_channel[0][0]

    #各botチャンネルで特定の期間以上経過した投稿を削除する
    print("bot")
    tmp_log_msg = delete_channel_message(bot_channels, 1)
    log_message = log_message + tmp_log_msg
    
    print("unimportant")
    tmp_log_msg = delete_channel_message(unimportant_channels, 14)
    log_message = log_message + tmp_log_msg

    requests.post('https://slack.com/api/chat.postMessage?token=' + SLACK_ACCESS_TOKEN + '&channel=' + LOG_CHANNEL_ID + '&text=' + log_message)

    print("done at " + str(datetime.now()))

if __name__ == "__main__":
    main()