A base has to be installed because the sub floor has a lot of deflection between supports (joists).
Plywood can be used but it still has deflection as well a movement.
Seams expanding and shrinking are were the tiles should let loosen first. The next place is where the plywood deflexs the most. Not a good system.
Some tile guys use Wonderboard. They come in and install the Wonderboard; after the board is down in minutes they are laying floor tiles.
The good new is that the mastic, which holds better to the Wonderboard, better than plywood the tile to the base hold good.
The bad news is has deflection doesn’t get the tile the seams will. In 15 SF areas you will have 16 lineal feet of seams.
The next place is where the Wonderboard deflexs the most.
Expanded gal. steel installed on top of that with cement pushed in to the expanded steel and a depth not less than 3/4″.
This makes a bridge over the entire floor. No deflections and no seams to worry about moving or separating.