Back in April 2011, @dmkcee announced the CodeGolf.SE First Periodic Premier Programming Puzzle Push. This proposal was so promising and interesting that even Jeff Atwood took note!
The 1p5 tag was introduced to classify this new venture of code challenges, as a bit of a meta-tag. Today, this tag has a meager four questions and 0 followers.
These four questions are not just random low-quality posts, however. They have accrued:
a total of 111 votes, for an average of 27.5 votes each (13, 19, 33, 46)
a total of 9.5k views, for an average of ~2.4k views each (580, 1k, 4k, 4k)
a total of 47 high-quality answers for an average of 12 answers each (5, 3, 19, 20)
a total of 24 stars for an average of 6 stars each (4, 0, 12, 8)
The tag's first two questions were posted in quick sucession shortly after @dmkcee announced 1P5, and they were the most well-recieved. The most recent two questions were asked just a week later, and received far less of the the gold dust enjoyed by the originals.
[...] we didn't even retire the tag 1p5. It has "encouraged" off-topic questions at least once. [...]
My assumption is he's referring to questions posted under that tag which were deleted / closed / untagged.
TL;DR At the time, 1P5 was a very effective effort on the part of @dmkcee and the community at large to improve site participation and quality, but...
...skip forward 5 years to today: PPCG has graduated, and we're looking like a very successful site (98% answered, loads of awesome questions per day, etc), so we no longer need 1P5 because it helped us grow. However, what should happen to the tag and its questions? It's both a meta-tag and nearly pointless from the tiny number of ancient questions with it.