I'm not aware of anything that depends on the SecureBoot status of a device, except possibly Widevine DRM for video streaming, but I don't see how that would be unique to ARM, disabling SecureBoot is also usually required on x86 for running alternative OSes without Microsoft's signed shim bootloader.
There is no Widevine support for OpenBSD anyway, so it's never been possible to watch Netflix. Fortunately, there are solutions for that...
Incidentally, Fedora Asahi Linux for Apple Silicon (ARM) got Widevine (hence, Netflix) working without any form of SecureBoot, they wrote about it recently on their blog.