Comment by SideburnsOfDoom

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.