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.
If anyone is looking for a great wallpaper app that supports a good company, Wallaroo from Icon Factory is absolutely fantastic and supports good devs that have made great software for a long time.