It's very broadly used. For example: - Apple uses it for their app to website association files - OpenID Connect uses it for connection discovery - security

It's very broadly used. For example: - Apple uses it for their app to website association files - OpenID Connect uses it for connection discovery - security.txt is usually served from .well-known - JSON Web Tokens uses it for Web Key Sets to verify public keys - LetsEncrypt uses it for its ACME HTTPS verification protocol. There's probably more, but these are ones I've personally used.