MSI B450M Mortar (non MAX) 3600 MHz RAM support with Ryzen 5 5600 - CPUs, Motherboards, and Memory - Linus Tech Tips
Dirt cheap Gigabyte B450 S2H is surprisingly capable of 3800 MHz RAM at 1900 FCLK (even official site only lists DDR4 3600 as a maximum supported frequency) : r/overclocking