It's Tomestones of Philosophy that are going away completely. They already in a previous patch were made useless and tradeable for their replacement, Tomestones of Mythology. In the next patch they'll be eliminated (so you need to trade any leftovers for Myth before then).

Myth are the next to be made useless and tradeable for their replacement, Tomestones of Soldiery. Essentially the same thing will happen there. Whatever used to be bought with Myth will instead be bought with Soldiery and your remaining Myth can be turned into Soldiery. And the new stuff to be released at that time will be bought with some new currency. So we've gone from Philosophy & Mythology, to Mythology & Soldiery (with Philosophy as a leftover just to trade for Myth), to Soldiery and something new (with Mythology as a leftover just to trade for Soldiery).

It's a repeating pattern designed to ensure that each major patch's new gear is only available with its new content, by releasing a new currency type each time for the new gear. There's always a new currency for the newly added gear, the second newest currency for previous gear, (with the third newest currency becoming obsolete and replaced with the second newest currency).