It's actually quite worrisome, many projects exclusively have their troubleshooting or support on Discord now what's going to happen years down the road when all those Discord servers have closed or no longer active and the invite links expire this is going to be a vast knowledge base that's just lost to the world
No, there was MSN in the 70s but communication was made through a machine called Microtron. They are largely lost to the world due to being made from degradable PCB plates.
Another way to think about this is that those projects that have a more structured approach to documentation have a better chance at lasting longer, attracting more contributors, and making more lasting impacts
If it's open source and the license allows it, I wouldn't consider that stealing. If a fork gets more popular than the original, then it either addresses a major missing feature of the original or is simply more active. If this displeases the original dev, they can hopefully work it out with the maintainers of the fork. This is a feature of FOSS, not a bug.
If you host a forum, you can easily access the database to move threads into some kind of archive if you no longer want to host it. It could also be moved to another server. Stuff like that.
Using a proprietary service instead is just a bad idea.