A 300 clarification (again) No, they are not. The slots disappearing is what would require a citation. One cannot be provided because this does not happen. It's not possible to prove a negative.

As far as actual play/expediency, I completely agree with you.

Your example of donning and doffing a +casting stat item is a perfect explanation for why you don't lose spell slots. Say you removed a +stat item while you had some spells remaining. With it off, you would have no spells. By this token of logic you could cast your final spell with it on, take it off, and then put it back on and upon doing so, gain bonus spells from the ability boost again for unlimited spells. Since this is not a known exploit, bonus slots do not work this way, and the inverse (having your casting stat lowered temporarily) would also not cause you to lose the slots.

Circumstances where you do lose a prepared spell or spell slot, such as being energy drained, are always spelled out in the rules. Having your mental score lowered in general, such as through ability damage, makes no such stipulation, therefore you keep your normal amount of slots even though you might be unable to use your highest level spells due to not meeting the minimum ability score.