I don't remember expiration dates either, but only became DM starting in 2015, so I would totally believe they appeared previously, then went away, and have now reappeared.

I came up empty searching my own pix (I thought I might have one because, a couple times I have sent them to Twitter agents back in the day when they were actually helpful; I also once loaned/traded one with a friend who was in-flight and wanted to reward a FA but didn't have any on them; and recently had a very helpful airport employee on a trip where I didn't have any with me but texted them one when I got home*) but Google image search has a hit from a DL blogger that shows them w/o the expiration date.

At any rate I have 3 left and only one DL trip left before year-end so guess I'll give them out then