It is my understanding that the 3600x cpu can support 3600mhz ram. Does a b450 board or any motherboard for that matter limit or determine ram compatibility when reaching these speeds? Or is it strictly cpu bound?
The motherboard does have an impact on the ram speed you can hit as well as the cpu - if you were running on a 300 series chipset board you would be limited to ~ 2933 max on memory no matter what cpu you use. That said B450 boards are typically much better than the earlier 300 series so 3600 mhz is possible on those boards with a Ryzen 3000 series cpu.
What I would say though is it does somewhat depend on what memory kit and what exact board you have - certain kits tend to work better with Ryzen (typically memory based on Samsung B die memory chips). If you are unsure I recommend going to your motherboard makers website and looking up the Memory QVL for Matisse cpu's to be sure.