I think the point here is that anything that can be indexed by search engines or archival crawlers would be better, so not Matrix either. Forums, for example. Like what happens if the Discord community gets deleted due to whatever circumstances? All of that gathered knowledge will be just gone with no way to recover or search for it.
What I did was say I'm deleting my WhatsApp account in [insert sensible time] and if anyone wants to contact me, to use Signal. Lost contact to some, but many followed.
While it still works, you can use old.reddit.com to get around those walls.