Telegram groups do not have end-to-end encryption, so you can join a group by clicking on a group link, similar to a forum. Signal groups are end-to-end encrypted, which conflicts with the forum concept.
Discussion
Got it! Good point. We need Signal more. Forums should be sites like Stackoverflow that can be indexed by search engines. They should not be locked behind walled gardens.
Perhaps because mobile phone screens are much smaller than computer screens, discussions that originally took place on web forums have moved to Telegram, which has a chat interface, especially its groups with thousands of members. This is just my personal opinion.