Why is the request denied

I don't work for Aternos but I've had an account since 2014 so I've picked up some useful information over the years

From what I've seen, you need to look for mods or plugins from what they call reputable sources, that means somewhere like curseforge (example for mods as that's what I use) because when curseforge (ect) hosts a mod or plugin, they do their own verification to make sure they aren't distributing a bitcoin miner ( or some other malware ) to their users.

Aternos relies on things like this to keep their own security budget small and as such can't afford the risk of adding something that could compromise their whole system, if you can do some research and find out where Aternos sees as reputable for plugins, then convince the creator of the plugin to try and have it hosted there (easier said than done) you have an easy chance to convince them to add it eventually, without that, I seriously doubt they'll take that risk no Matter how many people ask about it