Do you have a link to where Gecko removed it?

Based on just the usage in https://www.chromestatus.com/metrics/feature/timeline/popularity/791 removal would almost certainly be smooth, so it comes down to what other browsers still support this.

markdittmer@, is it already trivial to get information like this from the API catalog? Is it running somewhere yet?

I'm seeing it being present still in my FF nightly.

Re #2, my bad, I must have made a mistake.
I will edit #1. 

Removing this would (could) have the nice bonus of eliminating the only use of SVG{,Animated}Boolean. (Some of that may need to be kept if we want keep animation support though.)

It's also still in Edge, which means all major engines. I've filed https://github.com/w3c/fxtf-drafts/issues/114

I mentioned to Jing today that it would be valuable to expose his
experimental catalog sooner than later, even if only internal at first. No
concrete timeline on that yet though.