I think (just a guess here) if you are able to host it now you should be able to host it as long as Aternos exists, unless you get asked to stop too...
But (another guess) if your pixelmon lasts a certain period of time (a month maybe) then you are probably fine
I also realize money is probably a concern to you guys as well (from people who pay for perks), and if there is a drop of people paying for perks on pixelmon servers that might have to be removed, because I can imagine whatever it is that's giving you the ability to host pixelmon servers isn't free...
Also, I see some other users have looked at this, if u have ur own questions regarding this topic you can ask them too if u want