Would be fine if the person is having to BYOD as the phone is already theirs (which is how I took it at least). Obviously flashing a custom ROM of any sort without approval on a device that you don’t own, and would have to give back to the company is a bad idea. The same would apply to installing a different OS on a work PC (which tend to have the BIOS locked to prevent changing things or being able to simply boot an install USB). But I think managed phones with the level of being able to wipe the device remotely would also already be blocking access to developer options. Along with other stuff needed to unlock the needed stuff for flashing other ROMs (though I haven’t flashed a ROM in a while and never had a work phone so I might be wrong).

But if the company tries to take “ownership” of your personal device that you pay/paid for without providing a company owned device. Then it would be best to try and find a different employer, and/or use an old phone you don’t use or get a low-end/cheap phone while working for them. But only go with that option if they provide a SIM/eSIM for service that they are paying for.