Old powder?

Time flies!

I've got a half pound of ~20 year old powder abandoned from attempting to work up a load in the 90s.
Still smells and looks okay. Passed the match test.

Any issues I should be worried about?

I used up a can of BEYE last year that I am certain was older than I am. (Price sticker said $1.69. IIRC, powder cost around $7-8 per pound when I started loading in the late '70's.) No problems. As long as it smells OK and looks OK there is no reason not to use it up. If there has been any minor deterioration over time it might slow the MV a bit. At 50yds there was no POI shift between it and the same loads with a new can. I get quite a few partial cans of powder from wheeling and dealing, auctions, estate sales, gifts from widows, etc. If it smells wrong or looks wrong or it's not in its original can (or even if I don't like the look of the guy I got it from) it's fertilizer, but 3/4 of what I come on has been properly cared for and quite usable.

I noticed that I had to reduce some loads with some old BEYE due to it drying somewhat and not weighing as much per volume. I suppose it would be possible for some to adsorb moisture and gain weight too.

I had a couple of old cans of IMR 4064 and it worked just fine. Load a few see how it works and then dispose of it the proper way down range!!!!

When I first started reloading in 2005 my mentor Peter gave me some BullsEye that was made in the early 60's worked great

Still got my Unique, Herco, and Red Dot from the '70s and they all work just fine.