[DaiPlusPlus]

I think part of it was because around the same time that Chromium (which Electron is based on, thusly inherited by Slack) added baked-in support for capturing the user's screen and individual application windows - so Slack had no incentive to spend money to build their own native screen-capture and sharing code when they could simply wait and do nothing and then get it for free.

...but that still doesn't explain why Slack otherwise seems frozen-in-time: none of the significant end-user usability and quality-of-life issues I've had with Slack since I started using it in 2016 have been addressed (e.g. we can't right-click a message to get its context-menu: you still have to hunt for that awkwardly-placed "..." menu button).

See https://slack.com/release-notes/windows - this is their changelist for their Windows release and most of the releases since 2020 are described as "tweaks", was "tinkered with", or is "tuned-up" which, or "minor security updates" - which, as any npm user will tell you just means they ran `npm update` and not much else (well, besides the bare minimum of automated testing).