If you combine larger libraries from pre-eleven with post-eleven libraries you will see the different worlds (Qt as a prime example), small libraries can easily be enhanced with smart pointers etc. so do you don't have much of a clash. After eleven most features are nice things, which make things a bit nicer here or there, but don't change the code as much as reliance on moves and smart pointers do with eleven. Also upgrade from C++11 to C++17 is as painless as a compiler update can be. (Meaning: in some corner case you will probably hit some compiler optimization caused issue, but for most code it's a non-issue)
C++17 was backwards compatible with the venerable C++11 in almost all ways except for the removal of some library constructs few people actually use and have long been replaced by superior models, like `std::auto_ptr` and the old binder functors.
