Shadowrunners in Planescape?

The only way to avoid being detected by a power while sneaking into its realm is if you have an artifact made by a higher ranking power...

Yes, but bear in mind that powers are not nearly as strong when outside of their realms than they are while within it. Even quasipowers are impossible to surprise when within their dominions (unless you have a major artifact capable of masking your presence of course)

The ready availability of divinatory abilities is an "issue" with D&D game mechanics, at least in earlier editions. (I don't even have the 4e rules, so I'm not sure if it's still the case with those.) All sorts of divination effects are available, and depending on the campaign, may be absolutely commonplace. All sorts of potential plotlines are potentially ruined by something as simple as a detect evil or detect lie spell -- and true seeing is that much worse. Anyway, in a setting with powerful, commonplace access to divination abilities, infiltration is that much harder to accomplish.

Of course, in a low-magic campaign it's probably going to be a lot easier.

Anyway, the idea that "No, you don't get to kill a deity. EVER." is something that came into fashion in Planescape, but wasn't necessarily always the case before that. I would guess that the "No, you don't get to kill a deity" thing came about because the PS designers were trying hard to make absolutely clear that PS is not as hack-and-slash a setting as many other D&D settings (particularly those prior to Planescape) tended to be. It's a major Planescape trope that there's always something that's more powerful than your sword, no matter how many pluses it has.

If an individual DM wants a deity to be killable by mortals (or spied on by them, or whatever), I have no problem with that.

I'm really digging the idea of a Shadowrun'esque game set in the planes. You could get around the "Gods see through everything" problem by introducing a campaign-specific magical items that hides a possessor from the sight of any power. Maybe they were created by the Athar or by a god of Trickery/Chaos and then distributed (or stolen and dispersed). If the Athar were the ones that created the item, you can be sure they're an extinct faction by the start of the campaign since every power likely would come down on them like a hammer (though, they could then make for really interesting Mr. Johnson-type characters). Possessing one would probably be cause enough to have all the local religious types start crying for your head.