Is Aperture compatible with High Sierra upgrade process?
IF you bought Aperture from the Mac App Store, you should be able to re-download it from there from the Purchases tab, even though it’s no
  longer available for sale. (However, I never did, so I can’t confirm
  this.)


I can confirm this is possible to download on High Sierra still and that it is in the Purchases tab.

If you can't find it you may have downloaded it on a different account or it may be hidden, you can unhide purchases from the My Account link.