I think protocols for transferring BTC off-chain are on-topic (otherwise lightning wouldn’t be okay to talk about?) – even if it were about purely custodial wallets, things like proof of liabilities/reserves would be useful and interesting to discuss, I think.
I would have expected “high-level explanation of signature scheme” to go naturally in “Protocol design” fwiw.
There’s the option for adding a #cashu tag, or sub-categories (Implementation/cashu) which might help people either find discussions or easily skip over discussions that aren’t widely relevant. So that should make it easier to allow a broader scope without being too annoying.