A bit of rant but I hope this can fix some of the discrepancies I have found between free Nginx and the Plus one:
If I want to configure (minimum amount of) proxy connections to keepalive then I have to use upstream ... { ... } directive, but by using that nginx changes how upstream proxy url is DNS resolved. Within docker this can actually cause problems when the nginx container starts sooner then the upstream service - afair to fix it one needs Nginx Plus because in free upstream DNS resolution "mode" can't be configured.
I suspect that this might get interesting (in the way that one might want to grab popcorn for). I don't know the current numbers but a sizable amount of web servers use nginx to at least some degree.