Free Pascal changed the string type some time ago to make it encoding aware (i'm not really a fan of the idea, but it was done for Delphi compatibility which is considered important by the FPC developers). I only had to change a handful of lines in my 10+ year old code (at the time, now it is older) to make things work (all were about treating the string as a byte array and manipulating the memory directly - i just changed the type to RawByteString which provides exactly that functionality).
AFAIK that was the biggest "breaking" change they introduced by far. In general have code from 2007 that compiles out of the box and this sort of stability is why i stick with FPC (and C) despite it being messy sometimes.
If you make a mistake when making a language or API you should make sure whatever fix you come up with will keep the existing code working, most common way being that the old API is implemented in terms of the new (even if slower, things will keep working) or in the case of languages, new stuff that can conflict with existing code can be opt-in (Free Pascal often uses compiler submodes for this).
Yes, this makes implementing the library/language harder but it is going to be a bit of extra work for the implementors in exchange for avoid A LOT of work for the users (especially when you consider all the combined time wasted in porting Python 2 to Python 3).
