These two attributes are in Chromium, Gecko and WebKit, but not EdgeHTML.

Use counters:
https://www.chromestatus.com/metrics/feature/timeline/popularity/786
https://www.chromestatus.com/metrics/feature/timeline/popularity/787

There are 24,256 hits in httparchive for 'screen.availLeft', so even though this counters could be inflated by enumeration, there's probably a large chunk of real usage that would make removal very painful.

Standardization seems like the first thing to try:
https://github.com/w3c/csswg-drafts/issues/1090