To use the /delall command, the userbot must have access to the whole chat history.The reason for this is simple: if it always started deleting from message ID 1 and you used the command twice, the second time it would try to delete the message it deleted the first time, again.This would result in the userbot being apparently inactive for a long time, while it is in fact trying to delete already-deleted messages.To solve this, the userbot gets the ID of the first existing message and starts deleting from there: that’s what it needs the history for.

If you don’t want to make the history available, you can use /delfrom in reply to the first message instead.

https:///delall_bot?start=history