There's a number of RPC calls currently that don't need to know anything about the blockchain or the users wallet, and also don't need p2p network access:

• createmultisig
• signmessagewithprivkey
• verifymessage
• validateaddress
• combinepsbt
• converttopsbt
• decodepsbt
• decodescript
• finalizepsbt

Arguably also:

• decoderawtransaction (but createrawtransaction needs the node)

There's also a few being proposed:

• deriveaddress Add deriveaddresses RPC util method #14667
• encodescript RPC method 'encodescript' #14476

We could call the new thing bitcoin-util. The deprecation notice for bitcoin-cli method would simply be Use "bitcoin-util method" instead.

A bigger issue is that some application may depend on calling bitcoind via RPC, perhaps on a different machine. A few things we could do:

1. take a very long time to deprecate
2. add a parameter to bitcoind that adds all utility functions via RPC, like e.g. bitcoin-cli util-deriveaddress
3. let bitcoin-util run its own RPC server on a different port

However (2 & 3): maintaining RPC wrappers around utility functions is some overhead, and extra complexity.

Ideally bitcoin-util comes with a library that's easy to include in ruby gems, npm modules, etc.

One additional benefit of a separate utility is that you can switch between testnet and mainnet without restarting the node.