Apple is disabling downgrading across all of iOS, and starting to do the same with MacOS. So you need to keep old hardware to run older MacOS versions, and it's only a matter of a few years before Tahoe is the latest OS you can run on your Mac.
Oh, I must be clear here: I'm not considering M1 Macs or later, since Apple closed the ecosystem with Apple Silicon.
What you did is a downgrade in what's called the supported OS.
However, if you decide to downgrade to Catalina on an M1 Mac, it's not possible — Big Sur is the earliest version that runs on Apple Silicon.
Anyway, you cannot downgrade to a macOS version older than what your Mac originally came with. So if you buy a Mac now, Tahoe will be the minimum option.
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) :)
Apple is disabling downgrading across all of iOS, and starting to do the same with MacOS. So you need to keep old hardware to run older MacOS versions, and it's only a matter of a few years before Tahoe is the latest OS you can run on your Mac.