Slack has resolutely stupid browser blocking. My old Mac uses Safari v9. Slack, of course, wishes to block it for reasons. If I modify the user agent string to

Slack has resolutely stupid browser blocking. My old Mac uses Safari v9. Slack, of course, wishes to block it for reasons. If I modify the user agent string to something else, I’m unblocked and Slack functions perfectly.. IOW, there is no necessary reason to block the old browser: they do it out of sheer capriciousness.