On the Fairphone 3/3+, to be precise.
The Fairphone 2 comes with an unlocked bootloader which can’t be locked.
That’s the case on the Fairphone 3/3+, too.
Unlocking as well as locking the bootloader will force a factory reset for security reasons. For a new install with a wiped data partition this would be of no consequence to any user data, but it might be troublesome later on, so if the device supports relocking when running /e/, it’s better to make up one’s mind about this at the time of a new install.