RDs don't go out of adjustment. But you're problem is classic for a bent hanger. Since all adjustments are internal to the RD (except for cable trim), if the hanger gets bent inward, all the adjustments would be sifted inward, so the low limit no longer protects against overshifting.

Actually, index derailleurs tell you in advance there's a problem, but most people don't read the signs properly. As I said, if the hanger moves, everything changes together. That means the trim is also shifted inward. When this happens most people say drat, and promptly readjust it so the shifting is right. This doesn't have to be one obvious big change, but a bunch of small adjustments over time, and so not set anybody to wondering what's going on. While the trim gets corrected, the limit eventually becomes off enough to cause the fatal overshift.

Hint. While there's debate about cable stretch, we do know that they don't shrink. So if you find yourself readjusting trim outward, there can only be one cause, --- a bent hanger.

EDIT---

I misread the original post assuming it was the Low limit which accounts for 99% of serious RD problems.

Rereading, and seeing it was the high or outer limit, the answer may be much simpler. The limit may have been set a bit wide all along, but the RD may have been hanging on the cable in high (as it does in all other gears). Over time, the trim might have eased outward very slightly allowing the RD to move farther out, without the limit able to prevent it.