TL;DR
MKBHD is shutting down the Panels app at the end of this month, citing issues with finding the right development team fit.
You can no longer buy collections, and you must download existing wallpapers before the app is removed.
Users will receive automatic pro-rated refunds for active subscriptions, and the app code is promised to be open-sourced after shutdown.
It was split with the artists of said wallpapers, and was also kind of a gallery app thing IIRC.
But hey, as a Finnish saying goes, “It isn’t the one who asks who is stupid, but the one who pays.”