Not only nested ifs. It's not even correct (doesn't check for activity existing). And it's not even pythonic (ask for forgiveness, not for permission). Just access the thing, catch the exception and be done with it.

I worked with chatgpt since I'm not a python dev, and this is what I came up with

from time import time

class PlaySession:
    def __init__(self, data: dict):
        self.guild_id = int(data['guild_id'])
        self.user_id = int(data['user_id'])
        self.timestamp = data['time']

    def is_longer_than_half_hour(self) -> bool:
        return self.timestamp + 1800 < time()

    async def resolve_member(self, bot) -> "discord.Member | None":
        guild = bot.get_guild(self.guild_id)
        return guild.get_member(self.user_id) if guild else None

    @staticmethod
    def is_playing_league(member) -> bool:
        activity = getattr(member, 'activity', None)
        name = getattr(activity, 'name', None)
        return name and name.lower() == "league of legends"

async def ban_for_league(member):
    await member.send("The 30 minutes has elapsed and you are still playing league, get banned.")
    await member.ban(delete_message_days=0, reason="playing league")

async def process_entries(bot, entry_dicts):
    sessions = [PlaySession(d) for d in entry_dicts if PlaySession(d).is_longer_than_half_hour()]
    for session in sessions:
        member = await session.resolve_member(bot)
        if member and PlaySession.is_playing_league(member):
            await ban_for_league(member)

There are two types of programmers.

// comment
if(condition) {
  // comment1
  if(condition1) {
    // comment2
    if(condition2) {
      printf("hello, world\\n");
    }
  }
}

and

// comment
if(!condition) {
  return;
}

// comment1
if(!condition1) {
  return;
}

// comment2
if(!condition2) {
  return;
}

printf("hello, world\\n");

And one is objectively correct.

Correct me if I'm wrong, but the script works faster with nested IFs since if it fails at the first one, it won't bother reading next ones, unless you kill/return from function.

All in all, this feels like readibility argument, not correctness or efficienty argument