Ephemera and arcana I've learned decade ago still serve me well today, even when it's outdated people mostly either reinvent stuff so I can recognise new stuff as a variation on an existing thing or build on previous solutions so I know the details and circumstances that lead to some developments and this lets me understand new things better as well (puts it in context).
I think the usual back and forth about developer interviews may have this thought as an underlying assumption; specifically people who can solve a red/black tree on the whiteboard at the drop of a hat must be filled to the brim with useful but potentially obscure knowledge. (Obscure to those who haven't majored in CS or a related field.)
It would be interesting to submit an ask hn "What is your so there I was doing x, y, z when being able to answer obscure interview question seventeen saved the day" story. (That's not well worded but you get the idea).
Personally, I like this one.