The feature test proposed in https://gerrit.wikimedia.org/r/218955 (T102318) should be tested in production by running a sampled experiment that executes the proposed feature test and reports back to EventLogging whether it passed or not.
This will help gain confidence that this is an accurate reflection of our current status quo and a stable reference point from which we can move further.
If the total percentage of traffic passing the feature test (e.g. after a week) is below a certain threshold we can look at the misses and identify browsers we may be cutting out unknowingly. Then we'll decide whether we need to support them, or to document them as no longer supporting Grade A (since they'll naturally get the Grade C experience).