Past discussions are invaluable for providing context. The worst thing that can happen to a (technical) discussion is new people repeating points that have been resolved already while the discussion has since moved on.
Admins and regulars alike may waste time and screen space repeating past conclusions, something a quick forum search avoids.
I don't necessarily agree with that. On IRC you could have a specific rule listed explicitly in the topic and newcomers would still ignore it. The idea that they would trudge through megabytes worth of text to find the info is... optimistic.
Your only hope is to have a curated, wiki-like resource that you point newcomers at, maybe with an IRC bot to easily reference it from the chat.
I think people will generally read the log up to one day in irc/telegram/discord. Log of one day in these type of applications can be thousands of messages in a big group. Require everyone to read every message before sending a message is just unrealistic.
That actually done quite well compares to other message apps. The search query can be multi tags (text content, author, mention, is a file, contains link, before date, after date, at which channel) applies at same time. If you know what did you want to search, you can probably find it quickly.
Past discussions are invaluable for providing context. The worst thing that can happen to a (technical) discussion is new people repeating points that have been resolved already while the discussion has since moved on.
Admins and regulars alike may waste time and screen space repeating past conclusions, something a quick forum search avoids.