They are likely safe to eat but it helps if you know how they were stored.

If they were stored, say in an attic or a car and exposed to high temperatures, they will go bad faster.

Yes - because I just ate one that expired in 2017

Yes.
Here's how I know: Many moons ago I worked for a cycling advocacy group in another city, and Clif Bar was a ride sponsor. Somehow we wound up with CARTONS and CARTONS of excess Clif Bars that we couldn't even give away. The local food bank only took so many. We gave them to every volunteer that walked in the door. We included them in SWAG bags. We tried. Anyway, we wound up eating them off and on in the office over the course of a year past the expiration date. It was only when I finally opened one that had actual mold on it that we tossed them.

Probably safe, but likely stale tasting.

Probably, but I wouldn't take the chance. Alternatively, open one up and if it doesn't smell or taste stale, it's probably fine.

Are clif bars that expired a year ago but are individually wrapped and in the original box still safe to eat? I've heard mixed responses about this.