ARM has no management engine that we know of but the microarchitecture is proprietary so we can't be sure what's hiding in there. RISC-V is open source but the implementations can have proprietary add-ons. POWER9 looks good and is listed by the FSF as "fully free", Raptor Computing offer some blob-free mainboards but once you add a graphics card or a hard drive you add blobs to the system. POWER10 uses proprietary firmware for the memory and it looks like this will continue from now onwards:
https://www.devever.net/~hl/omi
tl;dr: we're all ****ed, there is no hope