Every now and then I return to the game Gravedigger hunting down the last 2 skeletons I've never found for those achievements, but I haven't seen hide nor hair of a single skeleton in quite some time.

Are they temporarily absent, permanently retired, or is my luck just REALLY BAD with finding them right now?

I played the game yesterday and remember uncovering part of a giant skeleton. Not sure which one, though.

I saw the giant skeleton in the last few days too. Failed to uncover him fully but he was definitely there. I play Gravedigger most days and have sometimes gone long stretches without seeing the skeletons so I think it's just a matter of luck. Stick with it, you'll get them in the end!