• Proposal (Proposal: deprecate and remove feature-exists #3749)
• Deprecation
  • Update official spec (Update the spec for the deprecation of feature-exists #3860)
  • Dart Sass (Deprecate the feature-exists function dart-sass#2235)
  • Tests (Update specs for the deprecation of feature-exists sass-spec#1986)
  • Website (Document the meta.feature-exists deprecation sass-site#1148)
• Removal
  • Update official spec
  • Dart Sass
  • Tests
  • Website

From sass/sass-spec#1939 (comment):

In practice, essentially all the new features we add fall into one of three categories:

1. New built-in functions or mixins, which are easy to detect using other sass:meta functions.
2. Language-level features which are relatively easy to detect on their own (for example, first-class calc can be detected with calc(1 + 1) == 2).
3. New syntax which can't even be parsed in implementations that don't support it, for which feature detection isn't particularly useful anyway.

This, combined with the fact that we haven't added a new feature to meta.feature-exists() in years, suggests that it's not useful anymore and should be removed.