This is on top of #20286. Only the top commit https://github.com/bitcoin/bitcoin/commit/11c22742150f44d28d15fc85a5fd5fd8fb7c157f is relevant to this PR. When the reqSigs deprecation period ends, this PR would need to be reviewed/merged.
This PR is meant to look at what goes away once we can get rid of the depreciation process for reqSigs. ie using IsDeprecatedRPCEnabled in core_write.cpp required (in the current implementation approach) a change to the build system. However it's temporary and these changes are only for as long as the deprecation period lives, as shown in this PR
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