I am not sure if you can permanently hide the fallback boot entries in the systemd-boot menu.
You could remove the associated conf files in /efi/loader/entries but they will be regenerated at the next kernel update or if you run reinstall-kernels command.
You could however disable the generation of fallback kernels by setting:
NO_DRACUT_FALLBACK="true"
in
/etc/kernel-install-for-dracut.conf
This will take effect next time the kernels will be updated.
Or perhaps running the reinstall-kernels command will make the change at once.