Generally true although 429 is often used for rate limiting so a back off and retry is appropriate. 409, 412, 428 may also be retriable depending on the specific semantics of the given situation. 421 apparently shows up commonly in HTTP/2 connection reuse and is retriable. 423 and 425 too potentially.

It would have been nice if there was an actually grouping of retriable and not retriable but in reality it’s a complete mess.

But at a minimum beware of 429. That’s not a permanent outage and is a frequent one you might get that needs a careful retry.