Is there a document that explores the backwards compatibility of these (and related) changes?
Particularly for open source software it's hard to just say "from next version well use 7.4 min" so anything that can be added and fallback gracefully on older versions is handy.
That's kind of the inverse of what I'd like to see.
Similar to browser docs what you're often interetested in is the latest things and how far back you can use them, and where they will degrade gracefully further back.
Those mostly assume you have old code that you want to make work on new versions, not that you want to write new code that takes advantage of as much new stuff as possible while still working reasonably well on old versions.
It's understandable that they would prefer that everyone just upgrade faster, so it's probably not something they want to encourage (both in documentation and in making features degrade gracefully if that requires any tradeoffs.).
So these new features are backwards compatible in that old code that has no typed properties will continue to work.
This is a minor release and PHP tends to be almost completely BC even between major release versions so it's unlikely you'll face any issues.
If you want to start writing code that uses these features in an existing library then you can branch a new version that depends on PHP 7.4, and define that as the requirement in composer. That way users with older PHP versions will pull older versions of your library.
Particularly for open source software it's hard to just say "from next version well use 7.4 min" so anything that can be added and fallback gracefully on older versions is handy.