Old Macs can certainly be downgraded. iOS doesn’t allow it though and they pulled the latest security update which fucking sucks. And if you buy a M5, Tahoe is the only OS that’s available.
I have nothing against old Macs and MacOS, but I certainly won't be buying anything since the Apple Silicon switch, because now only Apple controls which OS you can run.
The requested correction was on the "iOS has been locked down on downgrading since forever" part and another reply had already provided one for M5 being sold (though only pros seemingly) :)