Hello,
I have a unused Xojo license that I’d like to reinstall on a old pc, sadly I’ve lost the original installer. Is there a way to check up to what Xojo version a license can go? There is zero data on the licenses page, just an expiration date(14th April 2018).
If you look here Xojo: Account Login it shows the release date below the titles, your licence should work on any version with a release date before your expiry date, so 2017r3 in your case, 2018r1 came out on April 17 according to https://documentation.xojo.com/Resources:Release_Notes
If the release notes page(http://developer.xojo.com/release-notes) is accurate i missed 2018 by a few days 
Not that I would use that, 2018r1 was pretty buggy if i remember correctly.