[immibis]

Tier 1 providers means the Internet backbone. These are the networks whose core business is interconnecting the whole world. Think Level 3 and Hurricane Electric. It makes zero sense for a Tier 1 provider to deploy reverse route filtering.

(Tier 2 providers are those with lots of interconnections, but not the whole world, just regionally. Tier 3 providers are those who just buy wholesale internet service from another provider and don't have many if any other interconnections.)

No sane provider restricts UDP. You might be thinking of one of the more obscure protocols like SCTP.