Caddy (web server written in go) used to do something like this, basically it was ad-supported (injected ad headers) in a non-commercial free-as-in-beer pre-com

Caddy (web server written in go) used to do something like this, basically it was ad-supported (injected ad headers) in a non-commercial free-as-in-beer pre-compiled version and had a paid commercial version. If you compiled it yourself there were no ads or commercial restrictions though.