Locking the bootloader after installing /e/OS is possible on more devices than just Fairphones 3/3+ and 4.
In general locking the bootloader after installing /e/OS is possible for a device if the command line install instructions for the device available via https://doc.e.foundation/devices include this step.
How to avoid bricking the device (if that risk exists) should be included in those instructions, too, the Fairphone 4 instructions are an example for this.