This was added so that some add-ons wouldn't show "legacy" on about:addons. With the resigning of everything to external add-ons and the switch to Firefox 57 and legacy add-on only we should be able to remove this.
As of 57, that pref only applies to the UI when "extensions.legacy.enabled" has been flipped to true (in a build that honors it). In that case, the exceptions pref is used for a bunch of test pilot addons and the default theme. Test pilot should be okay now but there's still the default theme. We could: - replace the pref with a hard-coded exception for the default theme - remove the pref and let the UI show a bogus "LEGACY" badge next to the default them - remove LEGACY badges altogether - do nothing for now I vote for the last option
Ah good to know the first part. I agree with your vote. So let's leave this in the "stuff to clean out later on" bucket.