This BIP is based on the BCH OP_CHECKDATASIG work, as well as postings from the bitcoin dev mailing list in this thread. Some differences appear due to the activation of bips 340, 341, and 342 (taproot) since those were developed.
OP_CHECKSIGFROMSTACKVERIFY is a NOP upgrade available in all script types. OP_CHECKSIGFROMSTACK is an OP_SUCCESS upgrade available only in BIP342 tapscript.