In the storage areas of the fly shop I help operate I found dozens of Orvis, Cortland, and Scientific Anglers leaders from the early 1990's through early 2000's. All were stored away from light.

I have been using a lot of these leaders since I found them in 2009. We don't sell them.

I have found that most of these leaders were just fine though a few of the Cortland leaders did seem to have weaker tippet section.

Most of these could be straightened just by giving a good pull with the hands. For the rest, I just tape them up taught on a wall using blue painters tape. They usually relax over night.

On a few, yes I have pulled them through rubber - but only the butt and mid sections. This may weaken the mono but as long as these sections are still stronger than the tippet all is good.

Some of these leaders are now 20+ years old and are still fine. I use them a lot for local fishing for bass and panfish and stocked trout. I also use them when fishing smaller trout waters.

For old leaders I find with weakened tippet sections I just cut off the factory tippet and tie on new tippet material.

When I fish somewhere where i might have a consistent shot at trophy fish I do use new leaders.

The technologies used to produce tapered leaders has improved over the years so newer leaders may have improved characteristics like more supple tippets and better knot strength and strength for tippet diameter but these old leaders work fine for me in many situations.