People think of free software as merely open-source software, but it ain't like that. It is easy to understand why FOSS isn't merely OSS, but I want to enter into a different aspect that should define FOSS but doesn't.
If you've read the rants from websites like digdeeper.club, you might know what I'll talk about, that being customizability, extensible software, personalization, etc. This is not merely changing the color of a program, but how it interfaces or functions as a whole without the need of completely rewriting the source code as an user. I'll mention examples of software that are very customizable or extensible, though they aren't all FOSS but I'm merely using them as examples:
Windows XP, (Before v53) Firefox, any core utils (since many of them can be used to manipulate standard input and print standard output).
As example of software that can't be customizable to the user's preferences:
Windows 10 & 11, newer Firefox, Chrome, Android, iOS.
These examples should highlight what I mean; Windows XP isn't that customizable as I would like but, when compared to Windows 10 (an operating system where you can't even change the color of your taskbar), Windows XP & 7 end up looking like lego sets and Chrome is a particularly outrageous in limiting the user's control since nothing in that browser can be changed.
I don't think that people should change what they call FOSS based on this "subjective" standard of personalization, but that developers should seek to make their software open for customization beyond just making the source code available for modifications. In modern day, even programs that are referred as FOSS fall in this trap, such as GTK4, wether intentionally or not. In this point, older software shows itself superior, not because it gives anything special to the user or any outstanding privileges, but because they merely do not seek to restrain them.