No, you can't use
fastboot boot to boot into a custom recovery if the bootloader is locked (as Jerry Chin noted in a comment).
Android Security Internals says in Chapter 13: System Updates and Root Access (in section Recovery, subsection Custom Recoveries):
A custom recovery is a recovery OS build created by a third party (not
the device manufacturer). Because it is created by a third party, a
custom recovery is not signed with the manufacturer’s keys, and
therefore a device’s bootloader needs to be unlocked in order to boot
or flash it.
And in Chapter 10: Device Security (in section Controlling OS Boot-Up and Installation, subsection Bootloader):
In order to ensure device integrity, consumer devices are shipped with
locked bootloaders, which either disallow flashing and booting system
images completely or allow it only for images that have been signed by the
Thus, if you try to use
fastboot boot twrp.img, the bootloader will find out that
twrp.img is not signed by the manufacturer's key, and would refuse to boot into it.
And just in case we are not sure whether
fastboot allows bypassing the restrictions that a locked bootloader introduce, we also have in Chapter 13: System Updates and Root Access (in section Bootloader, subsection Fastboot Mode):
Commands that modify device partitions, such as the various flash variations, and commands that boot custom kernels, such as the boot command,
are not allowed when the bootloader is locked.