Solemnity combos with a fair number of cards.
Phyrexian Unlife and
Decree of Silence are probably the two most notable and have already been called out, but I'll add
Delaying Shield and cumulative upkeep cards like
Glacial Chasm. I feel like the majority of its use cases are as part of a pillowfort strategy to keep protection pieces around longer than they're supposed to be. The more proactive use case is with undying / persist creatures, who can be sacrificed repeatedly with it out. It's also a powerful hate piece if you have decks in your meta that use +1/+1 or experience counters... although it doesn't actually remove the latter, so I wouldn't count on it as an answer.
I don't think counter-based strategies are common enough to justify playing Solemnity just as a hate piece, but there are plenty of decks that can make use of it as part of a soft-lock. That generally won't win the game by itself, but it can buy enough time to find an actual win condition.