TTLs apply to UDP packets as well as ICMP or TCP. In fact, TTLS apply to all types of IP packets. It just so happens that it's easiest and fastest to send UDP

TTLs apply to UDP packets as well as ICMP or TCP. In fact, TTLS apply to all types of IP packets. It just so happens that it's easiest and fastest to send UDP packets instead of ICMP or TCP, because you don't have all the connection handling, and you don't have to worry about the underlying layers sending out retransmission, etc.... That's also why QUIC is an improvement over HTTPS, and is based on UDP.