Someone asked off-list if this query were related to a proposal. It is, to Markus Scherer's L2/12-108. That has been added to the the list of 'stability' guarantees at http://www.unicode.org/policies/stability_policy.html 'Property Value Stability', Version 2.1.0+. The redundantly retroactive version number confused me - I thought it was an earlier stability guarantee.
Unfortunately, the proposed (and implemented) restriction was on the decomposition mapping property, not on decompositions, so it would be entirely possible for U+E4567 to have a canonical decomposition to three non-starters, e.g. <U+0F71, U+0F72, U+E4568>. Such a character would also foul up the technique of collating FCD strings without normalisation by extended the set of collating elements, and would be an even worse nuisance than U+0F73. (That's why I put it in the loathsome plane!) Richard.
