HOORAY! I installed watchOS26.4 on my Ultra (not provisioned for cellular) and went for a run without my iPhone. Siri called out my splits!
HOORAY! Before I'd installed watchOS26, Siri called out my splits during runs and walks. watchOS26 broke this--after I installed watchOS26, Siri didn't call out my splits during runs and walks. I raised numerous bug reports via the Beta program, called Apple Support, and complained to Apple. The bug reports went unanswered, and the first guy at Apple Support replied with, "What are splits?". The second Apple Support person was no more helpful. I'd given up after about eight months.
HOORAY! It appears that Apple has fixed this, and the watch does not require an internet connection for Siri to call out splits during a run!