+1 for the feature 
This risk of incorrect behavior due to typos or lib API changes has been bothering me for a while, and I've definitively already encountered the issues you mentioned in your pitch a couple of times in the past.
In fact, I'm pretty sure there were some past comments here in the forums (or was it in bugs.swift.org back when it was a thing?) mentioning these shortcomings a while ago, and suggesting that we should need developers to use a dedicated keyword to be explicit about those. (sadly can't find a link to those past comments/bugreports right now, but I don't think it made it to pre-pitch or pitch stage, mostly comments on existing thread… will report back if I find the link)
EDIT: Found some past discussions about this here via a quick search. Didn't read them all in details, but could be relevant to check if some past info and discussions could be helpful:
- Mark protocol methods with their protocol
- Keyword for protocol conformance
- [Pitch] Requiring proactive overrides for default protocol implementations.
As for the keyword name bike-shedding, would that make sense to reuse the required keyword for this?