On Tue, Feb 15, 2022 at 01:37:43PM -0800, Jeremy Rubin via bitcoin-dev wrote:
> Unfortunately, there are technical reasons for sponsors to not be monotone.
> Mostly that it requires the maintenance of an additional permanent
> TX-Index

Alternatively, you could allow a miner to include a sponsor transaction
in a later block than the sponsored transaction by providing an (SPV)
merkle inclusion proof that the sponsored transaction was a part of a
previous block on the same chain.[1]

This does raise the vbyte cost of including sponsor and sponsored
transactions in different blocks compared to including them both in the
same block, but I wonder if it mitigates the validity concern raised by
Suhas Daftuar in the previous sponsor transaction thread.

-Dave

[1] Bitcoin Core stores the complete headers chain, so it already has
the information necessary to validate such a proof (and the
`verifytxoutproof` RPC already does this).  Utreexo-style nodes might
not store old headers to save space, but I presume they could store a
merkle-like commitment to all headers they previously validated and then
have utreexo proofs include the necessary headers and intermediate
hashes necessary to validate subsequent-block sponsor transactions.