At the moment, the feature flag for the SBE plan cache is disabled even in the "all feature flags" build variants (see here). This was necessary because we only wanted to turn on the SBE plan cache in build variants where SBE is also enabled.
SERVER-59178 will turn on SBE by default! This means that we should have the SBE plan cache enabled in the "all feature flags" build variants. In order to make this happen, we should simply remove it from the blocklist. At the same top, we should scan etc/evergreen.yml and get rid of any places where we are explicitly enabling featureFlagSbePlanCache, as this will now happen implicitly.