Earlier quoted context omitted.
I can find Slack conversations from years ago just fine, also link a lot to Slack in our ticketing system which is very useful. So for me Slack has become a second memory for technical and other details.
No useful threading in Slack, which is still one of its silliest issues. You must be using a paid version of Slack to see messages from a long time ago, as they have a limit on the no-cost version. This probably makes you dependent on your employer. When one switches between channels in Slack, oh boy is that think laggy! Sometimes I can wait for 5s, before the thing has switched, on a fairly modern CPU and up to date…
Sure, it could be improved, but so can anything.