Here is an excellent little single-purpose Mac utility: Unclack automatically mutes your mic when you are typing, and unmutes it when you stop. That’s it; that is all it does.
This was apparently released months ago, but I was only introduced to it recently, and it has been a very good thing to have. I am a loud typist pretty much always — it is a bad habit, I know — and this little utility means that I do not have to remember to mute and unmute my mic during online meetings. However, I have also discovered that I speak more while typing than I realized, thanks to this utility.
This is, for me, a perfect addition to my work-from-home software toolkit, and it is free. Recommended.