Continuing the discussion about noinput / anyprevout

On September 30, 2019, Christian Decker via bitcoin-dev suggested revisiting the sighash_noinput proposal (BIP-118) and AJ's bip-anyprevout proposal given the renewed interest in eltoo and ongoing discussions regarding clean abstractions for off-chain protocols. Open questions that remain to be addressed include general agreement on the usefulness of noinput / anyprevoutanyscript / anyprevout[?], strong support or opposition to the chaperone signatures, and output tagging / explicit opt-in. Another important open question is whether there is a clear understanding of the dangers of noinput/anyprevout-style constructions. Christian Decker believes there is weak opposition for requiring chaperone signatures, mixed (weak) support/opposition for output tagging, and that anyprevout signatures make the address being signed for less safe. He also believes that being able to guarantee an address cannot be signed for with an anyprevout signature is valuable. With regard to receiving funds spent via an anyprevout signature, he notes that there are no qualitatively new double-spending/malleability risks. Christian Decker does not believe output tagging is necessary and thinks that users do not need to mark anyprevout spends as "tainted" to wait for more confirmations than normal before considering those funds "safe". Finally, AJ suggests having a public testnet where weird noinput/anyprevout cases can be demoed and tested to determine if any extra code/tagging/whatever is needed to keep exchanges/merchants from losing money.