Simple: A version specifier, or feature specifiers. Backward compatibility concerns vanish when I can opt-in to a newer spec. Old code keeps working, and new code doesn't suffer for legacy nonsense.

For example, the Circle compiler extends C++ with its `#feature` directive: https://github.com/seanbaxter/circle/blob/master/new-circle/...

Sadly, the closest I've personally seen to this sort of thing in widespread use is `"use strict";` in JavaScript, which is only a single binary switch. You can't, say, turn on a new keyword, disable a keyword, switch to a different incompatible version of some browser API, etc.

I encourage all language designers to include a feature mechanism in a forward-compatible way. Don't overthink the difficulty: It doesn't need to do anything at first, it just needs to not be a parsing error. Treat it like a comment. FYI, this is the same as having a version number or header size in a binary file format's header, which all sane formats have (there are a lot of insane formats out there...).