Do you need very strict Backwork Comapibility that prevents CI4 development?

In my opinion, if the developer does not extend the base classes, then such changes will not affect the performance of the application in any way.
On the other hand, if a developer extends the base classes by making changes or implements the framework's interfaces, then he (the developer) must in any case check the compatibility of his code with each release.

ps: Here array $routes = [] specifying a default value is pointless.