Up until now there hasn’t been any evidence of actual backdoors existing, just speculative nonsense. A real backdoor wouldn’t necessarily be so obvious as the XZ backdoor or god mode bit in the C3 Nehemiah Core and is likely to just be a cpu based vulnerability that can be exploited.
Nation states (NSA, GCHQ etc) have nearly unlimited resources to waste on finding them so that’s likely what they do. I also think these organizations are abundantly knowledgeable about the risks of inserting backdoors. Such as those backdoors being used against themselves and companies in which they’re supposed to protect.
When the argument of backdoors comes up it tends to be from lazy domestic law enforcement agencies looking to expand their powers and bemoan the existence of E2EE - Eg. “Going Dark” and the countless campaigns about child safety in an attempt to pull public heartstrings in their favor.
It’s also worth noting that with ARM there isn’t really a standardized approach like the UEFI specification, so they often have very specific (and not open source) approaches to things.
We really don’t have much besides the Raptor POWER arch, and maybe in the future something that is RISC-V based. Also ARM requires licensing and is patent encumbered. It’s not really any better than x86 in that respect.