I marinated chicken five days ago in lemon juice, seasoned salt, and garlic powder, is it still safe to cook in the oven?
This would be a bad idea.
Chicken should be cooked 1-2 days after refrigeration according to the USDA and other food safety agencies, and will tend to get noticeably slimy and pungent after 3-4 days in my experience. 5 days is really pushing it.
I understand the rationale for the question - lemon juice can kill the surface bacteria - but that's just the problem, salmonella bacteria don't only exist on the surface of chicken, they're crawling around the interior as well.
In order for this to be considered safe, you would need to be absolutely sure of all the following:
The entire marinade had a pH of significantly less than 4. Lemon juice is considered to have a base pH of 2-3, but that is fresh lemon juice and undiluted. Unless you actually used a pH tester, you cannot be 100% positive of this.
The marinade completely penetrated every part of the chicken that may have been contaminated. This is literally impossible for you to measure.
The chicken itself was relatively fresh before it went into the marinade - unless you got it farm-fresh, you don't know this for a fact, so even if the marinade completely penetrated the chicken, the interior bacteria may have already left behind some nasty protein toxins (which the marinade can't kill because they aren't alive).
If you had said 3 days, maybe 4, I would say, practically, that you'd probably be OK. But any more than that and you're playing with fire.
As the old chef's saying goes: When in doubt, throw it out.
If you are intent on eating it anyway then please, at least be responsible and don't serve it to any guests.
The marinade won't have caused any food safety issues of its own -- whether it has "overcooked" in the marinade is another matter.
If it's been in the fridge for the whole period, then I would go by the date on the chicken's packaging, just as if it had been the fridge that long without a marinade.