What I see in that article is exactly what the PHB says: That you have to have a score in the relevant stat equal to the level of the spell +10 to cast the spell. 11 for first level, 12 for second level, etc.

Ah! I see what you're saying. You are talking of using other rules for slot purposes like permanent abilities based on slot sacrifice, and such.

Nevermind. I just restated the obvious.

Or preparing a metamagic spell. (other than Heighten Spell)

Yes, you will have access to a slot of a level higher than your primary stat allows you to cast spells; for example, with an intelligence of 14, a 9th level wizard will have 5th level spell slots. She will not be able to cast any fifth level spells, but she will be able to prepare lower level spells in that slot, with or without metamagic. The slot could be used for a fireball, or an empowered fireball, or a maximised scorching ray, but not a cone of cold.

Even if the Wisdom loss turns out to be permanent, the cleric eventually can make use of the spell slot that contains the unusable spell (see the notes on spell preparation in Part Two on slots for details).

now that would imply that the slot is unusable for the time being, but it can be made back up in time, either through a restoration, level increase, magic items/spells that boose stats

but the next part to this article should clear things up a little more

Bob the 9th level cleric takes wisdom drain and goes from a wisdom of 15 to 14. He has not cast any spells today, so that 5th level spell he had memmed will sit there all day and be unusable - unless he uses that slot to spotaneously cast a lower level cure spell. At the beginning of the next day, he prays and he mems cure moderate wounds into that 5th level spell slot.

What the article meant was that the 5th level spell couldn't be cast. However, the next day the cleric could mem a lower level spell into that slot.

i don't think so.....the way it read was that the slot is unusable until you regain the stat necessary to cast the spell... which would also imply that it would be as if you never had access to that slot to begin with...the slot ceased to exist essentially. the way you described it is broken...and as a DM i wouldn't allow it...

but like i said, the next article in the series should clear this up... so i say let's wait and see what the article says