I haven't performed a single windows install since 2013ish (was it 8.0 beta?), but I'm saying this based on how often I see it enabled. Companies do it, and probably manufacturers too. I'm less sure about the install media, true.

Regarding secureboot, I went through the pain of configuring it under Linux (creating and importing my own keys), before realizing it was of little use without a TPM. Turns out both Windows and Linux can't "own" the TPM at the same time, IIRC (work laptop has a windows partition). I ended up learning my randomly generated >15 char disk decryption password by heart.

On my work laptop, on which I dual boot Arch and Windows, I've just signed MS's keys with my own key and disabled booting from anything else than the internal drive.

I'm not sure what you mean by "legacy mode", but I'd expect that to mean "BIOS compatibility mode", and that's not really related (apart from presumably disabling secure boot). I actually prefer UEFI, this allows me to avoid wasting time with a classic bootloader.