As discussed in #Shim-lock, this is not the case on UEFI systems with Secure Boot enabled. Follow EFI system partition#Check for an existing partition to find out if you have one already, otherwise you need to create it.
Installing to the fallback boot path will still work even in BIOS mode since it does not touch the NVRAM. When installing to use UEFI it is important to boot the installation media in UEFI mode, otherwise efibootmgr will not be able to add the GRUB UEFI boot entry.It is recommended to read and understand the Unified Extensible Firmware Interface, Partitioning#GUID Partition Table and Arch boot process#Under UEFI pages.
