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.