POSIX and its ecosystem is also full of this backwards compatibility baggage (I for example claim that X11 has a lot more outdated backward compatibility legacy than WinAPI - no surprise since it is much older). Just to give a few basic texts about this:
The reason why people tell this all the time with respect to Windows, but much more rarely with respect to POSIX and its ecosystem, is in my opinion that much more programmers have inhaled the latter.
The Unix-Haters Handbook: http://web.mit.edu/~simsong/www/ugh.pdf
A Tale of Two Standards: A text by a person (Jeremy Allison) who understands both WinAPI and POSIX: https://www.samba.org/samba/news/articles/low_point/tale_two...
The reason why people tell this all the time with respect to Windows, but much more rarely with respect to POSIX and its ecosystem, is in my opinion that much more programmers have inhaled the latter.