True that technically parts of the code have changed, that's always been true. But as Satoshi said "The nature of Bitcoin is such that once version 0.1 was released, the core design was set in stone for the rest of its lifetime."

You can still use ancient versions of core and they still work. You aren't forced to use any code you don't want to and because of how Bitcoin works, any fundamental changes to the code sufficient enough to actually alter the protocol, it would create a new chain and not actually alter Bitcoin itself.

-Zach 🧙