There are several versions of Deluge, including the 1.3.15. that ain't liste on the whitelist, this does not mean you can't use it, they just state that they can't guarantee the workings. If the new version is on a Blacklist, then it's something else
I have a tracker that dont support the version I'm on, used it for several monts, no issues
As to previous version, binhex/arch-delugevpn:1.3.15_18_ge050905b2-1-04 should be correct. keep in mind, several people are saying they loose their torrents when downgrading
If the downgrade don't start, it gives you some error message? Some log would be needed to understand why it's not starting