While some forums do lock information behind registration-barriers, it's rare, whereas every discord is locked behind a email+cell phone barrier. It's far, far worse for anyone who doesn't want to be part of your community or install an app just to be able to read the docs or download a patch or ask a question.
>every discord is locked behind a email+cell phone barrier
Unless it's changed recently users can enter Discord communities without an email via an invite link*, just choosing a username. It's what made onboarding so easy and similar to just joining an IRC server with a nick.
What's been ironic is since Freenode had their spam issues years back there was a push among many channels on that network to allow only registered users (ie: email tied to nicks) to post, which has continued over in the migration to the Libera network and I find really unfortunate for such channels of those networks as low friction and less permanence in ties to identity have been a distinguishing feature of IRC. (Obviously this doesn't affect IRC broadly but those are among the largest networks for tech communities).
Ah, just re-checked that community and apparently they must have phone-gated it recently, as I haven't seen Blender use that before. Perhaps they got hit with spam, which I know is the reason some add the requirement, even temporarily (which I suppose circles back to the annoying side-effects of spam).
This isn't just a server-specific thing. Discord can and will randomly lock you out of your account completely until you verify with a (unique) phone number even if none of the servers you're in require it. If you have a discord account with no phone number, it will probably happen to you eventually.
I have joined 2 discords in the last week (as lots of 'community support' seems to be moving there these days). I set a username but I haven't had to supply any other info - I can't post anything and there's a banner saying I need to claim the account to avoid losing access but it's definitely not every discord which requires a verified account.
Doesn't help with the issues around searching for data from outside of discord.
Discord randomly (e.g. when a black box ML algorithm scores you naughty or if you just happen to log in from an ip that someone "bad" used) requires you to provide a valid mobile phone number before you can access anything.
That depends on the forum. Most forums could be searched and posts found via your search engine of choice. Well, the current crop of forums (discourse) prevents that with being JS based, but I would not count it as a good forum anyway.
Mailing lists and newsgroups are the proper mechanisms for this. And you should be able to use them with a browser forum-style too.