Sure it's not perfect in a world where IP are not necessarily static (and a VPN may change it), but having to login again is not that bad, even more so if you are using SSO.
No good. Slack's biggest userbase is corporate, behind corporate VPNs. Many users have the same IP.
(Especially if it's a user who isn't in a corporate environment, where your public IP will probably differ when you connect to different wireless networks. On a corporate network you're more likely to retain the same public IP no matter which room or building you're in.)
If they don't, they should provide an option for corporate administrators to enable IP locking for session cookies.
Logging people out also isn't a minor inconvenience, especially for users who haven't figured out password managers yet.
I know this isn’t actually a major thing. It’s a 20 second process. Still a couple times a day is a couple times a day.