Maybe. Not deliberately playing the contrarian, but consider perhaps one of the largest, and longest running software projects, the Linux Kernel, which has existed for a long time now using mail lists and IRC. Most mail clients can filter mail quite well, and everything is in one place, easily searched etc and has open protocols.
Using something browser bases puts you into a position where you have to choose between one or two browser engines and suffer however they manage the CPU and RAM.
Teams hogged the RAM and CPU when I used it in the browser, for what wasn't much more than IRC, and a terrible message archive. Mattermost isn't much better at searching either, and it's mostly glorified IRC channels. The only niche is perhaps mobile users, who, could arguably also use an IRC client or browser based one at that.
I wish. It is common for communities to switch from Slack to Discord. I would rather prefer a self-hosted Mattermost server or Matrix but eh...