The 5K LG display was available and available to deliver recently, so the lack of stock has taken place in the last few days. The display list has not been removed from Apple's site, so it's not entirely clear whether Apple plans to retrieve the monitor or if Apple does well for it.
Apple removed the online listing for the 4K LG UltraFine display last month, and before it is withdrawn, the 4K monitor has been referred to as "sold" for some time, so Apple may be planning to remove the 5K monitor from the site in the near future.
The 4K and 5K UltraFine Displays were designed by LG in partnership with Apple and were designed to be used with 2016 13 and 15-inch MacBook Pro models, which for the first time supported Thunderbolt 3.
27-inch 5K UltraFine display equipped with a resolution of 5120 x 2880, P3 wide color range and 85W power supply for charging MacBook Pro. Apple sold it for $ 1,299. The 27K display with 5K UltraFine continues to be available from Best Buy, B & H Photo and Amazon, and is still available on the LG website.
Apple is potentially removing the LG displays, because they are a couple of years at the moment and because it has a new Apple-branded display in the works. Apple is developing a high-end throughput modular Mac Pro and is said to plan on a 31.6-inch 6K display to go with it.
The new screen is described as a high-end display that targets the pro-user base of Apple, and is therefore likely to be expensive and may not be a suitable replacement for 4K and 5K displays that are no longer available from Apple.