Only assert backwards-compat experiments

Allow new experiments to not have to touch the spec, and we now have the
added benefit of using the spec's list of expected keys as a to-do list;
once all keys from that list have been removed (because their associated
experiments have been cleaned up), we can remove the need to check for
backwards compatibility altogether.

spec/lib/gitlab/experimentation_spec.rb

@@ -2,11 +2,11 @@
 
 require 'spec_helper'
 
-# This is temporary while https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45733 gets rolled out.
-# TODO: clean this up once the MR has been merged.
+# As each associated, backwards-compatible experiment gets cleaned up and removed from the EXPERIMENTS list, its key will also get removed from this list. Once the list here is empty, we can remove the backwards compatibility code altogether.
+# Originally created as part of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45733 for https://gitlab.com/gitlab-org/gitlab/-/issues/270858.
 RSpec.describe Gitlab::Experimentation::EXPERIMENTS do
   it 'temporarily ensures we know what experiments exist for backwards compatibility' do
-    known_experiments = [
+    expected_experiment_keys = [
       :signup_flow,
       :onboarding_issues,
       :ci_notification_dot,
@@ -23,7 +23,9 @@ RSpec.describe Gitlab::Experimentation::EXPERIMENTS do
       :default_to_issues_board
     ]
 
-    expect(described_class.keys).to match(known_experiments)
+    backwards_compatible_experiment_keys = described_class.filter { |_, v| v[:use_backwards_compatible_subject_index] }.keys
+
+    expect(backwards_compatible_experiment_keys).to match(expected_experiment_keys)
   end
 end