I’m currently running an 11-46 cassette on my GRX 812 rear derailleur (which officially takes up to a 42). It shifts… OK… as long as I keep it clean. On long, wet, muddy days performance degrades noticeably, but here in Vermont it’s been worth the sacrifice with so many repeated steep climbs on every ride.
I used a Tanpan and XT rear derailleur a few years back for an everesting and it worked pretty well. I think the Garbaruk cage is another viable option to do the same thing.