Since you've essentially written all the code needed, can we get rid of purge? It redundantly iterates message history. We already have message objects from our own history search, so let's use those. That being said, one strange thing purge does is use a single delete strategy if messages are older than 2 weeks. I don't know if that's important.

Reason why it does single deletes is because you can't delete messages older than 14 days via bulk delete endpoint, I didn't include that edge case in the code as (at least to me) it appears unlikely for moderators to attempt and purge messages older than that.
That being said attempts of deleting the messages older than 14 days via delete_messages will raise following error:

discord.errors.HTTPException: 400 Bad Request (error code: 50034): You can only bulk delete messages that are under 14 days old.

Do you think we can do away with purge though?

I don't think purge is necessary considering we iterate the history either way, I am all up to rewriting current system so it doesn't use purge in separate PR as it isn't exactly the point of this PR

If the current message is older than until_message, then the current message should not be appended and deleted, as it's too far back. Something like this, but maybe you can come up with something more elegant:

if message.created_at < until_message.created_at:
    break

messages.append(message)

if message.id == until_message.id:
    message_ids.append(message.id)
    break