It sorta came back as Reddit. I was a huge Usenet junkie from the first time I saw it, but it eventually turned into 99% binaries and junk. All the interesting discussions in the areas I was involved in went to sites like HN, Metafilter, Reddit, Web-based forums, etc.
It seems like we are going through a period where everything that used to be its own protocol is now recreated using HTTP and JSON.
Hell, it may well be that the web browser has become the new X server.
I've never used Slack but their front page alone is enough for me to turn away. The first two words I see is "Product" and "Pricing". Nothing on the page hints to an open protocol so I guess it is proprietary.
A really interesting (and probably really hard) project would be a proper decentralised Usenet replacement, with proper identity and reputation control. It might actually be something that blockchains could be good at, if you could avoid the requirement to replicate the entire database on every machine.
It might be feasible to have a single blockchain for identity and reputation, and then have each 'newsgroup' have its own, referring to it; that way I should be able to reduce the disk and bandwidth footprint to something reasonable.
Coming up with an appropriate costing system so that spammers were priced out without also requiring real money to actually use the thing would be the trickiest part, I think.
I've been trying to come up with some metrics for the size of "traditional" Usenet -- Big 8 hierarchy, say, early 1990s. Gene "spaff" Spafford thought that 50k - 500k users was probably in the right ballpark.
Note that to gain access you needed to be student or faculty at a research university, or work for one of a handful of tech companies (and almost certainly within their engineering divisions), or for a government agency with access. Very limited independent options existed.
The Usenet Cabal who managed things, such as they were managed, was about the same size as Reddit's technical staff, if that.
And the system proved highly vulnerable not only to spam and crap, but to users not acculturated into the system itself, and behavior protocols. The Eternal September was a thing for a reason.
These days, I've found a free provider for my text group access, of which
there are several.