TIL that Ruby has mutable strings, and (until the announced change) even had them mutable by default (and the change only affects literal strings; non-literal strings are still mutable). Python has always only ever had immutable strings.
One of the things I’ve said I would do and never did is create a set of test suites for as many facts about a language as I know and then run it for every new release to see what I need to unlearn.
Most but not all of these were performance related. If it took a few days to run that’s fine. Major versions don’t come out that often.
Strings will still be mutable by default after the change which only makes string literals always frozen (which has been a file-level opt-in for a while.)
Only if you count 1.9.2 as the beginning. What is being talked about is Unicode by default and maybe Unicode tooling (i.e. can correctly iterate over emojis and not just bytes)