Shaptons are waterstones that use some sort of ceramic abrasive
instead of whatever regular waterstones use. They wear hollow just
like waterstones only more slowly, and should be flattened whenever
they get too far out. Flattening them does not reduce the rate at
which they cut.

Until Shaptons came along there was no confusion regarding "ceramic"
stones. These are ceramic through and through (as opposed to ceramic
grit in some sort of media). The coarse and medium grit ceramics are
made to fairly rough flattness tolerance - 0.010" if memory serves.
The fine grit ceramics are made to 0.001". They probably have to
throw out a lot of them because fines cost twice as much as mediums
or coarses. My understanding is that you cannot flatten a true
(non-Shapton) ceramic stone without compromising its cutting ability.
That observation was made during an old FWW review of various stone
types.

To restore most of the cutting ability of a non-Shapton ceramic you
scrub it aggressively with water, Comet, and a sponge.