1a) If you're presenting via webex/zoom never share your main screen (either share a 2nd desktop screen or single app).
1b) Try to always present without mirroring your screen. it's harder to follow with high-res, but that's an argument maybe your res is too high.
2) If using Powerpoint, use presentation mode (and preso-mode for energy settings so you don't accidentally go into lockscreen).
Barring the above, I go offline on both Outlook and Slack during presentation mode.
macOS has 'Do not disturb' mode which is activated automatically when mirroring to TVs and projectors (configurable), a feature very neat.
Absolutely. I used to care when facebook or tinder or whatever app dinged at me (I call out these two specifically just because they're the most recent ones I've disabled). Now, I assume the ding has nothing to do with me and should be ignored (and, then I turn off notifications or uninstall it, once I realize the app is abusing its place on my phone/tablet/computer and not respecting my time and attention).
As you write, I think applications who abuse their permission to interrupt the user lose their credibility and usefulness. It is just the equivalent of having a word processor putting advertisement on a printout. It renders it useless.