These are not standard and there doesn't seem to be a signal from standard folks that they will be anytime soon. They should probably be eventually removed from SpiderMonkey
Posted the site compatibility doc: https://www.fxsitecompat.com/en-US/docs/2015/conditional-catch-clause-support-will-be-removed/
I only meant to file the bug, I don't the decision has been made to the point it can be communicated yet. ni'ing jorendorff in his quality of module owner.
I think we should keep this extension. Some features are either incompatible with the ECMA standard, or an implementation burden, or both. Those should go. This is neither -- and it's been allowed in all JS versions for many years, so dropping it will cost. Not every language extension needs to die. It's a judgment call.
Just to note, that since we're using ESLint and ESLint doesn't support conditional catch clauses, I think pretty much all of the instances that Firefox was using have been removed. If we did want to allow it in gecko code, we'd probably need to find a way to extend the ESLint parser in such a way that we could support it, although I suspect that could cause more work than we'd really want to do.