That's interesting because I was just thinking the other day about how it used to be; for example, if you had a program that ran in Windows 95 and you moved (or more likely, were forced to move by changing computers) to Windows 98, stuff would be immediately broken. In recent years, Microsoft (I don't know about other software companies) has done a good job in making their operating systems backwards compatible.
But I guess there is a limit, which is reasonable, I suppose. OOTP 5 goes back to 2003 and those were the days of Windows XP, I believe. There has been a lot of water under that bridge since then.
There are indications that it is possible to downgrade from Windows 11 to Windows 10 on a new PC; for example,
this. But I would weigh the time, effort, expense, and risk of doing so against your love for OOTP 5. Is it worth all that?