I’m not sure about other servers, but it feels like on Skyfury, during the initial leveling rush tanks were readily available but healers was the major bottleneck for running instances.
It also feels like end game is over saturated with tanks.
Now, while leveling during Joyous Journeys, it seems like a lot of people have been leveling non tanking classes much more than before.
Of course, as someone who leveled my first two toons as tanks and now as a healer/dps, I may not have a very unbiased view of things.