Both Gigabyte and MSI motherboards had the cpu-less boot feature for flashing BIOS when I checked this past summer/fall. (I was checking for a theoretical 5600G build.)
For MSI motherboards in particular, there’s a switch in back you can see in the pictures on Newegg so you would 100% know it would have the feature. I’ll see if I can dig up a link to show you what I mean.
EDIT: The first MSI board I checked has the button, and I think they all do. For what it’s worth, I have an MSI motherboard, albeit for Intel. Steve from GamersNexus isn’t particularly complimentary toward MSI motherboards, but I like mine.
Check this out on @Newegg: MSI MAG B550 TOMAHAWK AM4 AMD B550 SATA 6Gb/s ATX AMD Motherboard MSI MAG B550 TOMAHAWK AM4 ATX AMD Motherboard - Newegg.com-sr--9SIB7RHGU46684-_-01102022
That Newegg link includes 12 pictures, and picture 8 shows the backplate. In that backplate picture you can see clear as day the “Flash BIOS Button” which lets you flash the BIOS without a CPU.
EDIT2: I assume gigabyte boards have a similar button in back you can look for in pictures. I just didn’t happen to when I checked last summer or just now.