Is it given that 6s won't receive iOS 13? I bought mine because it's the only one still having an audio jack for my car stereo, and the 7 and 8 add nothing for me, except the price. Though maybe it's telling Apple themselves don't sell the 6s anymore.
iOS 13 drops support for iPhones with 1GB of RAM. 5s and 6/6+ are out. The 6S/6S+, the SE, and above are supported.
iOS 13 will be supported on the 6s, just not on the 6/6 Plus (and below).