You may have run across these Legacy IP Only caution stickers at some event. I designed them in 2013 and printed 1,000 sheets of two and sent them off to assorted IPv6 folks I knew from various mailing lists. All of the artwork is available and several other people printed their own as well. They have been handed out at a lot of conferences and related events.
Here is where the idea was hatched. In April of 2012,
RFC 6540: IPv6 Support Required for All IP-Capable Nodes
was released. As of 2023, it is in the category of Best Current Practices as BCP 177.
I first learned about it in the tweet pictured. I immediately had the idea that
a caution sticker was appropriate.
I'd occasionally think about this over the next 14 months, but I finally
designed the sticker in June of 2013.
A decade after printing the first batch of stickers and the publication of RFC 6540, twenty-eight years after RFC 1883, and twenty-five years after enabling production IPv6 at work these are still in demand. I handed the last of the 2013 batch to a friend who was going to FOSDEM in January of 2023.