According to :
A few years ago some of the plumbing had to be replaced in the bathroom
of the house I grew up in, and part of this was to remove some mosaic
tiles that were put down about 1974 using a mastic. Man, getting those
babies up was a great deal of hard work, and now that ceramic tile has
entered a project of my own, I find everyone pushing thinset. Is it
really better? If there were a bit of floor flex, wouldn't a plastic
mastic tend not to crack as much as a thinset cement? And with a bit
of mastic coming up between the tiles, wouldn't it keep water from the
plywood (yes, right on the plwood!), rather than continuing to suck it
down as a cement would after going through the grout?
Now the floor was 3/4" diagonal T&G with 5/8" plywood, so perhaps
flexing wasn't all that much a problem (I've forgotten the exact joist
spacing & depth, but 2x10 16" OC seems to stick).
Still, with all the effort needed to remove these tiles, why's
everybody now down on plastic mastic?

With a sufficiently flexible floor, it doesn't matter what adhesive
you use, you'll be cracking the grout and even the tiles.

With 5/8" ply over 3/4" T&G, flex isn't going to be much of a problem.

However, modern tile installation uses a special plastic underlayment
that permits the _entire_ sheet of tile to float - much like centuries
ago where they floated the mosaic tile on an adhesive, over a layer of
sand. So not even expansion/ contraction of the subfloor will bother it.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.