apt - Find which versions of Ubuntu a package in a 3rd-party repository is available for?

I have a link to a 3rd party package repository, for example something like this:

http://eddie.website/repository/apt stable main

There is a particular package available from this repository (for example, one of the packages in this repo is airvpn).

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How can I find out which versions of Ubuntu this particular package (and, similarly, other packages from any other 3rd party repos) is built for ?

I'd like to do this to find out if software I'm using already available on 18.04, or which older versions of Ubuntu are still supported by the software. It seems like I need to look at the "release" files in the repository, how do I get to them, and where is the info I need ?