[mshenfield]

You're right, there is no obligation to maintain it. I think that misses the point though. The value in keeping the code is to allow the end user to continue to enjoy improvements in parts of the library that don't have breaking changes without upgrading those that do. You could continue to have security patches installed, for example. That value is much less when you don't do basic maintenance implement bug fixes and security patches.

[blueprint]

Unless I'm missing something… the answer to that problem is to (a) factor the code sufficiently to then (b) create an abstraction (interface) that backs out the concrete implementation to the specifically desired version/functionality.