You are moving the goal posts from OS limitations to limitations of old applications.
Unix applications written using old assumptions (e.g. ?14? characters max for symbol names in libraries, ?16? bit address space, assuming the C library knows of gets) can have problems on modern systems, too.
No one still uses Unix applications from the 1970s.
Lots of people still use Windows applications with the limitations mentioned here, either because they refuse to give up their 1990s Windows applications, or the weird little ISVs refuse to update their code.
Unix applications written using old assumptions (e.g. ?14? characters max for symbol names in libraries, ?16? bit address space, assuming the C library knows of gets) can have problems on modern systems, too.