So now the free official binary distribution doesn't allow commercial use but you can build it yourself from the source (which is licensed under Apache 2.0) and use it commercially for free:

"""

Q: Which license do I need?

A: If your company uses official Caddy binaries internally, in production, or distributes Caddy, a commercial license is required. This includes companies that use Caddy for research. The personal license is appropriate for academic research, personal projects, websites that aren't for profit, and development at home.

Q: Is Caddy open source?

A: Yes, it is. Caddy's source code is licensed under Apache 2.0, which requires attribution and stating changes made to the code when forking it, using it in your own projects, or distributing it. This website distributes official, compiled Caddy binaries, which are licensed differently.

"""

https://caddyserver.com/download not once mentions the fact that the server is open source or could be compiled from source for free. The entire page is structured to give the impression that commercial uses require a paid license. Other servers like nginx have separate websites for the open source and paid versions (http://nginx.org/ open source, https://www.nginx.com/ plus)

There's a link to GitHub right at the top...