Idk I generally think “magic numbers” are just extra effort. The main annoyance is adding if statements everywhere on version number instead of checking the data field you need being present.

It also really depends on the scope of the issue. Protos really excel at “rolling” updates and continuous changes instead of fixed APIs. For example, MicroserviceA calls MicroserviceB, but the teams do deployments different times of the week. Constant rolling of the version number for each change is annoying vs just checking for the new feature. Especially if you could have several active versions at a time.

It also frees you from actually propagating a single version number everywhere. If you own a bunch of API endpoints, you either need to put the version in the URL, which impacts every endpoint at once, or you need to put it in the request/response of every one.

I think this is only a problem if you’re using a weak data interchange library that can’t use the schema number field to discriminate a union. Because you really shouldn’t have to write that if statement yourself.