No, it really hasn't been. Solo content post-leveling has always been pretty much an afterthought with WoW, especially once they got instanced PvP rolling.
Yes, there has always been things to do solo, like farming materials, but they've never been particularly engaging or rewarding. Outside of a short period of time from mid-Cata (Molten Front) through the end of MoP, solo content has always been sorely lacking as anything other than a grind for grinding's sake.
You may be confusing the fact that WoW was less punishing to players as being solo-friendly, e.g. death penalties. It also included much of the quality of life enhancements that various games had been making up to that point and continued to add more as other games developed them. Both of these things made solo play more fun, but the heart of the game has always been dungeons/raids and, later, battlegrounds.