OpenResty is only available via Nginx Enterprise?
Definitely not, Kong (for example) is "OpenResty plus a management plane" and they're Apache 2: https://github.com/kong/kong#license
OpenResty itself is BSD https://github.com/openresty/openresty#copyright--license
I mean some features you get with haproxy are only available with either enterprise or OpenResty.
These days I always go for haproxy for reverse proxying.