[kazinator]

Without knowing which part you don't get, there is a risk I just repeat everything in a scrambled order!

The highlights are:

We have software (a programming language and its library) that is versioned in a simple, linear way: it goes from version N, to N+1, to N+2 and so on.

Users who are using version K now depend on some features. Suppose the behavior in version K+1 changes some of the features. The users will be rightfully unhappy; they upgrade to K+1 and things work differently, breaking their code.

To anticipate this, we can have a command line switch or environment variable whereby users can request "please emulate version K". Then version K+1 (and K+2, K+3 ...) will restore those behaviors which were altered starting in K+1.

This does not disable purely new features that don't break existing behaviors. For instance, if a two-argument function can now take an optional third argument, such that a two-argument call behaves exactly the same way as before, that won't be subject to emulation. A whole new function that didn't exist in version K is not going to disappear under K emulation.

This isn't a perfect strategy. Things can go wrong. But it's fairly decent.