The only reasonable design is to have two kinds of API keys that cannot be used interchangeably: public API keys, that cannot be configured to use private APIs,

The only reasonable design is to have two kinds of API keys that cannot be used interchangeably: public API keys, that cannot be configured to use private APIs, and private API keys, that cannot be configured to use public APIs. There's no one who must use a single API key for both purposes, and almost all cases in which someone does configure an API key like that will be a mistake. It would be even better if the API keys started with a different prefix or had some other easy way to distinguish between the two types so that I can stop getting warnings about my Firebase keys being "public".