The IO shield issue prevents L12S from being installed on the ASUS mobo in horizontal heat pipe orientation (bend to rear).
Workaround by reddit user u/catesnake: imgur.com/gallery/yM50rhD
L12S and T1 case compatibility is a separate thing, caused by multiple factors: L12S size, CPU socket placements on different mobos, and T1 side beam location.
The MB side beam overhangs the mobo edge section where you find CPU 8pin connector and fan headers. Reducing the airspace limit of this section to about 65mm in 2 slot GPU mode.
For AM4 mobos, with T1 flipped (fans on top), L12S in horizontal or pointing up (bend to pcie) orientations won't fit because the fins, which requries 70mm clearance, also overhangs that section.
If you are ok with not installing the MB side beam, there is a workaround.
www.reddit.com/r/FormD/comments/iaccvo/my_solution_for_having_the_nhl12s_in_the_right/
In order to have both the MB side beam installed and L12S fit in horizontal orientation, the MB side beam needs to have the option to be moved up. My rough estimate is ~16mm+ to cover all current AM4 mobos. However
that isn't a simple change.
The bracket for mobo standoff and riser holder is currently coupled to the GPU side beam for structure rigidity. The GPU side beam has to remain where it is. If it moves up in sync with the MB side beam then the clearance issue will still be there. Now you have the two side beams positioned asymetrically and that will have flow on impact to case fan mounting.
Personally, in the beginning, I wanted to do an optimal full air cooled build in T1 using L12S. Given that 2 slot high end GPU is almost non-existent, I've accepted 3 slot mode and I will use AIO to cool the CPU.