You may still see it because (and I can't speak for the Military discount in particular) when T-Mobile shifted to the no contract plans and significantly reduced service prices the idea was to have a unified pricing structure and not one for some and another for others. In most cases Verizon even with their discount was significantly more expensive than TMO without a discount. But for those that already had a discount TMO like most all pricing changes or plan changes grandfather's those already on a plan. For the discounts they required annual recertification that you were still eligible under the discounts original requirements, if no you lost it and were no longer eligible going forward other than the Aadvantage program you listed.
In T-Moble's "defense", that link you provided doesn't say there's a 15% discount.
These forums are over a number of years (I've been in 5 different cubes, personally, in my place of employment [which is not T-Mobile and may become AT&T if the DOJ approves] during the years of these forums [although, there was a reset when we went from the old style to Jive])
Now, there WAS talk that says "Government Employees" still get to keep their discounts. I don't know if that's still a thing, or if that's become "plan based" as well. In my opinion, Military is considered a "Government Employee." A T-Mobile employee will have to chime in as to whether this is the case or not.
I guess that you did not get what I am saying.
I attached the link to show that there is no longer 15% discount offered to military members who are new T- Mobile One plans.
Stevetjr mentioned above that the 15% discount is only applied to the older plans before T-Mobile unified its priciing structure.
The latest post in community asking about military discount is in May 2017 which service members are still confusing and asking for military discount.
T-Mobile did not really come out and said that there is no discount for new T-Mobile One plans.
In stead of 15% discount, service members will get a $25 reward card when they apply for the advantage program.
I have mine still from when I signed up for it.
heavily discounted plan doesn't get the discount added on top of it.
Right now I am paying $160 for 8 lines. I don't have my discount being applied, but it's still on my account.