Has anyone found a way to remove the yellowing on my older cake plates and pillars?

no -- egg white wash it and sprinkle it with sugar -- very pretty --

but i have heard of someone putting it in their pool and the chlorine whited them but i never did that

I like the egg white and sugar route.

Don't know the answer but I have been known to whiten and sanitise plastic chopping boards this way. It may work with cake plates and pillars:

1. Lay the board on a generous sheet of Cling Wrap.

2. Pour bleach over both sides of the board.

3. Wrap well in the Cling Wrap, adding extra Cling Wrap.

4. Leave in basin for an hour or two, or even overnight.

5. Rinse really well.

6. Put through the dishwasher.

N.B. Be extra careful with the bleach.

I like the egg white and sugar route.

Don't know the answer but I have been known to whiten and sanitise plastic chopping boards this way. It may work with cake plates and pillars:

1. Lay the board on a generous sheet of Cling Wrap.

2. Pour bleach over both sides of the board.

3. Wrap well in the Cling Wrap, adding extra Cling Wrap.

4. Leave in basin for an hour or two, or even overnight.

5. Rinse really well.

6. Put through the dishwasher.

N.B. Be extra careful with the bleach.

The problem with bleach is it's a corrosive and can eat plastics. Soaking pillars in beach or a solution that has a too high concentration of bleach may cause pits or other damage.  Test  something first that you won't miss if it does not work out.