While we have several newer installations running version 7 that we're very happy with, we still have quite a number of systems dating back several years that are still running version 6.
It is simply not feasible for us to convert these to version 7 since, as a testament to it's stability, these customers of ours just need something that continues to work just as it always has, that's it, and we must oblige accordingly.
That all said, we have been successful in upgrading Asterisk to version 13.X on these V6 systems, but not without headaches, since it was never itself upgraded to 13.X capability (canreinvite vs directmedia, SetMusicOnHold deprecation and so on).
I contacted Thirdlane to see if we could at least get a new maintenance release of version 6 that fully supports latest Asterisk and, understandably, they told me there would need to be interest from more than just a small few customers (I'm sure there's much more with the same issue as us).
Please let them know if you guys would like to see a small revival of version 6.
I am with you, I have 3 licenses that I would love to see some updates to. I am brought back to the website as our raid array died and I spent a long time to reconstruct all this from memory !