There are multiple factors, this is only one of them.
Add-on authors are free to use this if they wish.
Percentage formatting isn’t deprecated and is still the most used formatting method in Python’s own code-base (from a quick search).
Reminder that this isn’t a mechanical change we can simply apply to the existing code-base, it pushes us towards spending time refactoring code which we didn’t have plans to change for Blender 2.9x.
My own two cents- I’ve been using f-strings exclusively since they became an option. I find them far more useful when the variable is in the correct context within the string. Remember: readability counts! If you’re using variable names that are easily understood, having them bracketed inside an f-string means you can just ‘read the string’ left to right and understand exactly what you’re looking at by the time you reach the end.