If you intended RFC to be a reference to IETF RFCs you should now it originally stood for Requests for Comments before simply being referred to as RFCs. (link) I didn’t have to read your document in detail, but it seems reasonable and well thought out.
I was able to enable it on stable by toggling browser.urlbar.showSearchTerms.featureGate
Edit: Nevermind, it doesn’t work for custom engines, which suggests that it’s a separate implementation from mobile?