REST has become so dominant that people treat it as the natural way of building HTTP APIs — not a way, but the way.

I am 100% sure you have already found yourself debating which flavour of 4xx the API should return in a certain condition, if a given operation should be a POST or a PATCH and so on. This is because although REST philosophy makes total sense in theory, in practice any non-CRUD scenario easily fall into a gray area as you are probably talking about operations on an application's domain, but REST demands that you treat it like resources.