I've actually been able to redeem a couple of the old yellow undated chits. About 9 months ago I flew HOU-LGA and the FA took them and didn't care. Another time I had a FA literally laugh at me and say "we haven't accepted those in years" and but given her demographic I figured she wouldn't be friendly to the old yellow chits but decided to chance my luck anyway.
I only wound up with them because my mom was cleaning out old files and gave them to me since I fly WN more than her. I'll keep trying to use them until I run out. I'm not part of the class though, so no new old tickets for me.