Yes I have noticed they been absent for over a few weeks now. Even the 13" got down to only 2 models of the nTb but then they rebuilt the stock a week or so back but nothing for the 15". Very odd.
2015's are in high demand and Apple knows that. So, they pulled them because they would rather you buy any computer they sell that has USB-C only. Think about it, for every 2015 sold by Apple, that's a 2018 not sold and a step backwards for where Apple wants to go with their line of notebooks. People want the 2015 because of legacy ports and a tried and true keyboard and MagSafe and a glowing logo, etc. And of course, because it's still a great computer. But if there are none available, then folks will be forced to buy something else. Now all of this is speculation on my part, but if I was in charge, that's what I would do to help usher in the new and sweep out the old. Off the record, they probably have a large inventory of 2015's somewhere in some warehouse, but are either writing them off at cost and disposing of them, or passing them off "cheap" to third-party sellers like B&H Photo and other places like that.
The 15" Refurbished are back. All be it they are only 2017 models. To be honest there is better pricing from 3rd party suppliers for NEW 2017 than the 2017 refurbished apple is offering.