Well, it's not ActiveX, but Google has been known (and is still known) to use a number of non-standards (often, as you point out, could-have-been-standards-but-

Well, it's not ActiveX, but Google has been known (and is still known) to use a number of non-standards (often, as you point out, could-have-been-standards-but-weren't) in their own web properties, locking away non-Chrome users. Last time I checked, this was still the case with Google Hangouts, for instance, which used non-standard WebRTC. I don't know about Slack specifically, but third parties have been known to follow suit.