I set "Show Supply Drop Player": false, in my Inbound.json config file, but it still announced "// Mike // has deployed a supply signal at L13". Seems like maybe this config option is broken/unimplemented, or I don't understand its purpose?

Edit: For now I've turned off "Supply Signal Alerts" since I'm not sure I want the announcement at all anyway.

The 'Show Supply Drop Player' config option relates to Cargo Plane / Supply Drop alerts. Its purpose is explained in the documenation. For Supply Signal alerts, you can simply modify the lang file in oxide/lang/en/Inbound.json.
Changing:

"SupplySignal": "{0} has deployed a supply signal{1}",

To something like:

"SupplySignal": "A supply signal has been deployed{1}",

Should accomplish what you're after.

Thanks! It wasn't obvious to me that the setting applies only to the plane/drop alerts and not the call one - but I understand your thinking now, and will follow your suggestion.