[Update: Not really] OnePlus discontinues the OnePlus 3T 128 GB15
Apparently, a OnePlus rep said that the company stopped selling the OnePlus 3T 128 GB for a good reason: being a rather small company, OnePlus needs to focus its efforts on just "one flagship device" - obviously, the flagship device mentioned here is the upcoming OnePlus 5 that will be officially released sometime this summer.
While OnePlus confirmed the name of its next smartphone, as well as the fact that it's going to be launched this summer, it didn't reveal anything else about it. Anyway, rumors have it that the OnePlus 5 will be quite an attractive handset with features including a Snapdragon 835 processor, dual cameras, up to 8 GB of RAM, and a large battery.
Like the 3T, the OnePlus 5 could have 64 GB and 128 GB (or maybe 128 GB and 256 GB) variants. Hopefully, both will be available to buy from day one - we're saying this because, as you may know, the OnePlus 3T 128 GB was released months after the 64 GB model.
source: OnePlus via Reddit, Tweakers (translated)