The only valid reason for the guidance around URIs is to encourage proper usage of the REST verbs. If a request performs an action that is consistent with the client's expectation as per the HTTP standards then it really does not matter what the url contains.
Naming urls based on nouns makes it natural to create behaviour that is consistent with the intended purpose of GET, PUT, POST and DELETE.
When you put verbs in the URL it can become very confusing because often the http verb will have contradictory behaviour to the one in the URL. REST rules say you must respect the HTTP verb, but usually the url is more descriptive so it can be misleading.
The fact that browsers only support a subset of HTTP verbs is not really relevant because even when you do have full access to all the HTTP verbs, you still need to be able to model other verbs, like print, close, confirm, cancel.
You are absolutely right that people need to focus way more on content types than URL structure when talking about REST implementations.
Making your URLs refer to nouns is not a REST constraint, it is about encouraging people to fall into the pit of success.
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