O2-absorbing crowns - do they absorb other things?

I would be surprised if there is any merit to it, at least as far as my understanding of the science behind the oxygen absorbing crowns go.

O₂ can't simply be absorbed - it has to bond with another substance and oxidize it. The idea is that there is a substance in the crown that when activated by becoming wet will bond with O₂, causing the chemicals in the crown to oxidize rather than the chemicals in your beer.

Ideally the electrochemical potential favors the oxidation and not the reduction, so the redox reaction proceeds only one way.

If that's not the case and the oxidized scavenging compound is later reduced, releasing the oxygen to later oxidize something else, then perhaps that might later cause more oxidation of the aromatic compounds. But that would be a pretty silly design for a oxygen scavenging cap!

It appears that there are both homebrew and commercial issues with absorbing crown liners causing some issues. There is a tweet circa 2010 by Ray Daniels (Cicerone) claiming 2/3s of a loss of aroma within a couple of weeks.

I don't know how accurate this claim is; however, I can tell you that I use the caps with my beers, and if I'm drinking them within 6 months it doesn't cause much of an issue (even with IPAs). If you're planning on doing some long-term aging, controlling the temp of the beer in the bottle may be more effective at protecting the aromas (storage temp does affect the aromas in a great fashion).