I just took a look at Tower's site, and yes, the TT's are gone, rather sad, along with a good portion of all the other makes of engines. They had a really nice "green" .18 that was one of the first to disappear about 7 or 8 years ago. Others have commented it was a decent sport engine. Fox and Super Tigre I see "on order" or some other reasons that have persisted for a long time, telling me they'll probably never return. I've got enough Testors McCoys, Enyas, K&Bs, OS Maxes and etc. to keep me occupied for a while, so their in their absence won't be missed.
As the electric CL stuff gains greater maturity, I foresee electric as the future direction, to ensure the persistence of CL, but for that will probably be a while.