I run an AsRock Rack X470D4U, which also only offers USB 3 in its various permutations. It does not even have an internal USB 2.0 header. I'll give a brief history of my booting experiences:
When I first bought the board, I had the usual IT-tinkerer's collection of no-name flash drives, or brand name flash drives that were quite old (like pre-USB3 old). I tested every flash drive I owned using Unraid trial keys, and every single stick gave me similar results:
Some sticks simply would not boot AND
I had to physically change stick ports every time I rebooted my machine, and this only booted correctly most of the time.
Obviously, this all but eliminated remote management feature benefits, which were hugely impactful in my motherboard selection criteria. I bought my Pro license and lived with these reboot headaches for about 6 months, complaining all over the internet about how unacceptable it was that in 2019/2020 we still can't boot Linux from USB 3, and texting my wife from work to "please do the thumb drive swap thing" every time I needed to reboot using remote management.
Then my flash drive of choice (read: the last drive I used in my testing that happened to boot on the first try) failed catastrophically. I also happened to have every other flash drive in my possession filled with various sundry files or OSes. PLUS, SpaceinvaderOne (may the olde gods shine favor on his lineage til the heat death of the universe) just had posted his Flash Drive Testing video. So I bought a SanDisk 32GB Cruzer Fit USB 2.0 Flash Drive - SDCZ33-032G-G35, swapped my key and imported my flash backups.
It booted on the first try. My mind was blown.
It even reboots seamlessly without having to swap USB ports. Praise Linus.
All this to say, I think certain flash drives support booting better than others, and I think USB 3 gets an undeserved bad rep for it.