Pleased to announce refinements to the BIP draft for OP_CHECKTEMPLATEVERIFY (replaces previous OP_SECURETHEBAG BIP). Primarily:
2) Changed the opcode specification to use the argument off of the stack with a primitive constexpr/literal tracker rather than script lookahead
4) More detailed comparison to alternatives in the BIP, and why OP_CHECKTEMPLATEVERIFY should be favored even if a future technique may make it semi-redundant.
I believe this addresses all outstanding feedback on the design of this opcode, unless there are any new concerns with these changes.
I'm also planning to host a review workshop in Q1 2020, most likely in San Francisco. Please fill out the form here
https://forms.gle/pkevHNj2pXH9MGee9 if you're interested in participating (even if you can't physically attend).
And as a "but wait, there's more":
1) RPC functions are under preliminary development, to aid in testing and evaluation of OP_CHECKTEMPLATEVERIFY. The new command `sendmanycompacted` shows one way to use OP_CHECKTEMPLATEVERIFY. See:
https://github.com/JeremyRubin/bitcoin/tree/checktemplateverify-rpcs. `sendmanycompacted` is still under early design. Standard practices for using OP_CHECKTEMPLATEVERIFY & wallet behaviors may be codified into a separate BIP. This work generalizes even if an alternative strategy is used to achieve the scalability techniques of OP_CHECKTEMPLATEVERIFY.
2) Also under development are improvements to the mempool which will, in conjunction with improvements like package relay, help make it safe to lift some of the mempool's restrictions on longchains specifically for OP_CHECKTEMPLATEVERIFY output trees. See:
https://github.com/bitcoin/bitcoin/pull/17268 This work offers an improvement irrespective of OP_CHECKTEMPLATEVERIFY's fate.
Neither of these are blockers for proceeding with the BIP, as they are ergonomics and usability improvements needed once/if the BIP is activated.