"It really annoys me that increasingly its required to join slack / discord / telegram channel in order to connect with developers of projects I'm interested in."The Freenode IRC network is full of developers and users providing support for open source projects, and is still highly active.
For example, Freenode's #ubuntu, ##c, and #vim channels have about a thousand users each, and plenty of them are developers. There are plenty of channels for smaller projects too, usually with developers of those projects hanging around and answering questions.
Come join us! Sure, it's not Usenet, but it's not a proprietary walled garden like slack, discord, or telegram either. You can actually keep text logs of the channels you're in and IRC clients are pretty full featured. The only thing I miss from the proprietary competition is embedded images/video, the lack of which is sometimes actually a plus.