Since Linux has no concept of a base system, it's a stand-alone kernel with a hodgepodge of crap around it - this distinction makes no sense on Linux.
/opt is generally for software distros for which you don't have source; only binaries. Like commercial software packages. More common on Real UNIX(R) because most Linux users outside enterprise aren't running commercial software. You're putting your $500k EDA software under /opt.
Steam says hi.
On Windows, a common Steam library exists in Program Files directory, therefore not user specific. On Linux, each user has a separate Steam installation and library. I'm not sure why there isn't a common Steam library on Linux, but /opt would be a good place for it.
No reason this can't be done on Linux but since NT's security model is more flexible it's a lot easier to do so on Windows. You'd need to add dedicated users. (Running a Steam daemon as root would probably cause an uproar.)
Developers who knowingly reduce or disable default Windows security settings should be censured. Because in 99% of cases it is due to ignorance or plain laziness.
It doesn't "reduce or disable default Windows security settings" in a meaningful way if you say to yourself "that folder effectively is in ProgramData, but spelled wrong".
You should never hardcode the path since it can and has moved around, though MS has implemented hard links to legacy paths because most developers are stupid and against persistent better advice do it anyway. I've seen multi-million dollar software packages whose vendor requires it to be writable by "Everyone".
Steam was first released in 2003, three years later.
For 80% of grievances about Windows, there is likely a solution in place that no one knows about because they didn't read the documentation.
But Windows XP, which came out in 2001, inherited everything from Windows 2000 and more, and was used extensively for gaming.