What is this type of write-protection called?
AVR calls this feature "Protection Modes".  You boot into a "protection mode" that does allow rewriting some or all of flash, or into a "protection mode" that does not allow rewriting flash.


When you set a "protection mode" that allows writing some flash, you can boot into or jump into an address where flash-writing is allowed, and you can boot into or jump into an address where flash-writing is not allowed: on these processors you can't write the flash region you are currently executing.

One use of this feature on the AVR processors is to boot into a mode and address that allows writing flash, update the flash, then jump into the updated flash: that allows updates at boot time, but no updates after the jump into the non-boot code region.