Its carrier related with Text message signature. I believe Verizon and third party carriers have signature text messages only. ATT Sprint Tmobile etc should be getting it back with the new 6.0 Android update Marshmellow .
I don't have that option and I'm on AT&T
Maybe AT&T took that option off
Sent from my Samsung Galaxy S6!
in the settings on s4 it had a signature block so everytime I sent a text, it was there but not on this phone