You end up mastering a subset of the language. Typically whatever you use at work or in your free time. Whether that subset is 10% or 90% is kind of up to you.

There was an old joke about everyone using a different subset of C++ but like most jokes it's mostly an over exaggeration.

C++ (and C) have amazing backwards compatibility and there is a strong chance that code written 30 years ago will still compile, link, and run as long as the dependencies haven't also changed too much (I'm looking at you Win32, X11, Motif). That gives you the time to migrate code bases over time without the fear of being left behind and unsupported.

Even if, for some reason, you were writing C++98 for the last 30 years, it shouldn't be too difficult to make the jump to newer standards. The biggest change, in my opinion, is lambdas and move semantics, and it's not to difficult to apply those to an old code base (if you want to) and start modernizing it. Some people say C++20/23 is a new language but the fundamentals of C++ haven't changed too much and I'd say it's easier to learn C++23 coming from C++98 than it is to learn Rust coming from C++98.