I didn’t even know what this could do until I lost use of a finger recently, got a new mouse today, and really badly needed to remap its side buttons.
I can’t use software to remap because I already am, and Wayland doesn’t let you do that. Oh no, now wtf am I gonna do?
Never fear! KDE settings to the rescue! I almost shed a tear, for real. These little things can make such a huge difference. I just had to say something, so here it is.
Thanks, mystery KDE contributor(s).
If I am thinking of the right feature, this was work by @davidre merged here:
It’s helpful functionality even for folks without any physical constraints, and I bet David and others would be excited to know it has improved the accessibility of your setup 
I am glad this feature helps you!
It was SUCH a relief mate thank you SO much!
I just had this moment of realising that I had a technical problem which I likely could fix, but was going to be tricky, and definitely require a lot of online research and configuring stuff… and my right hand is pretty much out of service right now.
I was sitting there kinda groaning like WHYYYY…this is gonna SUCK… Sometimes life just wants to kick you when you’re down… and then my memory just went “Hey you saw this button in settings a few weeks back, maybe that does something?”. 10 seconds later I’m done. Absolutely magic.
It’s not the first time that KDE has given me a moment of “Hey wow that was really easy, someone went to some effort to make that so accessible, thanks random KDE person” but this time was just too much for me to keep it to myself.
On a technical note, I was especially impressed that it worked correctly as a momentary switch, too - as in, mousedown = keydown, mouseup = keyup. That fails a lot with these kind of things. Not here, tho! Well played!
Thanks also @johnandmegh for helping me give credit where due