Not a huge fan of this but I can understand the compatibility concern. I think we do need a plan to eventually match scintilla (i.e. have a consistent API too).
E.g. 1) Introduce `sci_get_selected_text_length2()` with the new behavior and deprecate `sci_get_selected_text_length()` now 2) Remove `sci_get_selected_text_length()` at some later point 3) Re-introduce `sci_get_selected_text_length()` at some even more later point
We could also introduce some mechanism where plugins declare a API TARGET. E.g. if they target 242 (or none, because it's new) they get the old behavior, if they target 243 they get the new behavior. Obviously this works only for cases where we can handle the behavior change at compile time and in header-only. At some point we could reject plugins that target too old APIs (always with good reason, not "just because").