This comes up often enough that it probably ought to be added to the list of commonly rejected ideas (which, in fact, I think has been suggested already somewhere). See the following links:
- override-like keyword for default implementations
- Mark protocol methods with their protocol
- [Pitch] Requiring special keyword to mark protocol implementation methods
- [Pitch] Requiring proactive overrides for default protocol implementations.
- [swift evolution] [proposal] Proposal to add "implement" keyword to denote protocol method implementation
- Require use of override keyword to override dynamically dispatched methods defined in a protocol with a default implementation