New devices in the US that have an unlockable bootloader and custom ROM support?
One Plus is the newer exclusion in the list of reducing number of new devices in the US which can run Custom ROMs. Right now, I only really know of the Pixel and the Nothing line of mobiles which can run Custom ROMs.
I believe ASUS phones have an unlockable bootloader - although the app for it currently doesn't work (several month long maintenance?) but is estimated to work again sometime Q3 of this year