I try to treat slack/teams/irc like the office water cooler. Sure, we can discuss work, make decisions or come up with solutions to problems, but it has to be recorded elsewhere, in an email or internal documentation or whatever for it to matter and for it to be official.
Nothing sucks as much as trawling through old chats to find some decision that was made ages ago.
Yes, while a "work slack" or "company teams installation" is more formal than the ones that I mentioned above - it is for doing your job, while on the job - it's really not a good store for decisions and other facts.
To be clear, the slacks for "former employees of $Company" were not set up by the company, but by those people in order to keep in touch with each other. The chat had no official relationship to the company. This is why these slacks were regarded as the online equivalent of the pubmeet.