I don't recall there ever being a formal proposal for Cloistered Cleric. What may have happened is that someone asked about it informally and got a discouraging comment about our general opposition to new base classes.
Over time, the general principle that we don't want new base classes has been interpreted more and more loosely. We've approved racial paragon classes (which aren't exactly base classes, but are more like base classes than prestige classes) as well as variants to barbarian and paladin. There is no formal definition distinguishing a new class from a tweak of an existing class; the paladin and barbarian variants we've approved are closer to existing classes than the cloistered cleric is. So precedent migth still be considered to argue against the cloistered cleric. But such precedent is not binding; I, for one, am quite open to any of the UA classes found in the d20srd and Sovelio-Sage versions of the SRD, because they are fairly widely recognized to begin with, and easy to find in sources that most of us use anyway. In addition, in the case of the Cloistered Cleric, while I do think of it as a substantially new class, I feel the archetype it represents is one worth including, and core-rules classes just don't fit the bill.
