Comment by px43

Large group chats should have an expiry of one week or so, maybe 4 weeks at the longest, otherwise it gets crazy. Unfortunately the best way to delete those old messages in your local chat is to delete the conversation and rejoin it.

Admins really should have a "purge all history older than X" option. They let you do it one by one manually, but it's basically impossible to automate, and doing it manually is super tedious.