Theoretically you could remove features by deprecating them (with warning) for a specific c++xx version, and removing them in the next. So if c++14 warned on some $BAD_FEATURE, and then $BAD_FEATURE was removed in c++17, there is still a migration path from c++11 to c++14, and eventually c++17 once all deprecation warnings are removed. (or even before, if someone wants to take the time to remove any remaining warnings in one shot) If the compilers were then able to support the exact set of warnings/removals per version by a flag, old codebased would continue to compile and theoretically would continue to move into more modern features as the next version would never actually break the code.

Probably would be nasty to maintain for the compiler developers though. Imagine having to do a bugfix for c++11 mode when you just released c++26 mode.