Marking Tiles will make a return in the next major update to the game (yes, it was a bug that it disappeared). Sorry for the inconvenience.
While we are on the topic of marking tiles I noticed when we still could do that we were unable to un-mark a tile once marked. I just thought I would inform you of this bug for when that feature returns.
I'm glad to hear it will be back. In very large fights with bosses similar to their troops, it helps to be able to highlight the boss, rather than describe where he is located before the timer runs out.