So long for muscle memory (oh and for consiseness, it's worse in French).
Anyways, that's the prime reason there's no list: either they want to change the commands willy-nilly, or they don't know them because that's whatever the model's learned.
Only saying "15 minutes" initiates a timer for that long.
This goes even deeper in the "undiscoverable commands" issue at hand.
"notify me in 15 minutes" feels natural and casual, and how I'd expect to interact with modern voice assistants. "set a 15 minutes timer" feels overly formal and redundant (it does not help that in French, a timer is "minuteur", so you repeat the "minutes" sound twice), and how I'd expect to interact with old voice assistants. This new one is just some hidden trial-and-error thing deep in Siri that's likely an engineer that likes cooking that added it as a shortcut.
Among the many shortcomings of Siri is that it seems as if it's not good with verbs. I've learned to avoid them as much as possible. Put another way, it's better with nouns, so I focus on them. I guess that's why
"15-minute timer" and
"15 minutes"
work well. But similarly, I wanted to use the stopwatch the other day. Not something I ever really do. Just saying, "Stopwatch" got it open. And testing now on some non-Apple apps also worked (in case Siri has some built-in pro-Apple bias). One was WhatsApp. The other was an app for an insurance and banking company. That one, just saying its name opened the contact card I have for the company. That's fair. Trying again, and saying "company name app" opened the app.
Of course, sometimes the verbs are necessary. But I've had more success when I could avoid them. Do note that I say all this using a Siri-only phone that is too old for any of Apple Intelligence that may get mixed in with Siri.
But the reality is it doesn’t work and users have to specifically learn the few things it can do.
Regarding the "notify" vs "timer", I had a very similar experience with a friend. I went to a bakery, and she asked me to get her some kind of pastry. To me, she meant some kind of bread. Queue confused faces on both sides when she asked where her stuff was. Sure, it's still in the broad "baked goods" category, just like a reminder and a timer. This was in France, both living in major cities 200 km apart. It's not like some extreme variation of English from the other side of the world.