I connect to Slack from my favourite IRC client (Irssi) after getting fed up with the memory that the official client used. It's not as retro-cool as this C64 port but it fits my command line orientated workflow better than the Electron app did and allows me to build my own client-side plugins in Perl.
Historic messages don't need to be cached in RAM if it's several pages above the proverbial fold.
However I'm not convinced it's just historic messages that's eating up the memeory. IRC clients aren't this bad even with thumbnail support like Slack. Whuchever way you look at Slack, it's needlessly wasteful.
That memory footprint is inexcusable even if he were literally on one thousand Slack teams. Functionally, Slack is more or less a shiny wrapper on IRC, and you can run dozens of IRC channels in a few megabytes, if that.