do we need a forward-backward compatibility plan?

Frank’s recent proposal on languages got me thinking: do we need to
adopt some policies, and add some sort of language to the spec, that
allows us to manage this better long-term? I see three categories of

  1. the CSL syntax and processing model (the most tricky)

  2. types and variables

  3. locale terms

Just an example: say we have processor written for 1.0. What should it
do with an attribute that gets added in 1…01?

Or we should we ban those changes except with full point releases?

Basic goal here is in particular to not have a bunch of forking of
style and locales files.