It's a bit of a meme about how Hakurei Reimu's gohei gets longer and longer with every game. (e.g. post #9147881, post #9146775, post #9149969, post #7730781 for examples)
With the announcement of Fossilized Wonders there's been even more art of Reimu's gohei, this time it merging with the logo (post #9143645, post #9139259, post #9153233).
Should this be a pool? We already have pools for Flandre's wings (pool #1628), Sakuya's knives (pool #21411), Komachi's scythe (pool #6222), Hecatia's hat (pool #10382), so I'd imagine it's not too much of a stretch to have a pool for posts about Reimu's extremely long gohei.