While I certainly sympathize with the situation you are in, the fact of the matter is what I have highlighted for you. The part is the word "technically". That word has, unfortunately, caused significant struggles, pain, and downtime for our clients and significant overhead for our support on items which are outside our scope of support already. This was one of the many reasons to force authors to verify and confirm what version their add-ons work for prior to allowing them to appear as such in the Marketplace. Then also to verify those versions on install for clients.
As much as I hate to throw anyone under the bus, it is also a little protecting you from yourself scenario too. Where clients have installed add-ons from far outdated releases, which just bring their site down. This may not be your scenario here but this is an extreme example that came up often in the past.
If the author wishes to provide you the files, you can certainly upload the add-on as a custom item to your installation still. Though, from what it sounds like, the author has abandoned the project so that may not be as helpful but still worth a try to reach out.
Ultimately, this has been discussed a lot on the community and while it has not fallen on deaf ears, as we truly value your input, it is not something which we are going to change at this time. I hope this helps clarify why this was done and I do want to say again, we do value your input here and it does continue to shape how we do things here.