Apple released an iMac with 5K Retina display late last year, but has said nothing publicly about any plans for an iMac with an 8K display. However, in detailing plans for 8K displays, LG said that such a device was coming later this year.

From the LG Display blog:

It has become clear that Japan is planning to launch an 8K SHV test broadcast and then promptly restructure the UHD service. Apple has also announced that they will release the 'iMac 8K' with a super-high resolution display later this year.

It seems highly unlikely that Apple would release a new model of iMac with an 8K display just a year after debuting the 5K model. It's always possible that Apple placed orders for 8K displays with LG, and the company just let something slip that they shouldn't have.

It's also possible that Apple is indeed working on an 8K product of some kind, such as a new standalone monitor. However, none of Apple's current devices support 8K output, and devices will require DisplayPort 1.4a in order to do so. Additionally, LG's only current 8K display is actually just four 4K displays, bundled as a single, 98-inch display.

It seems likely that LG simply made a mistake in the blog post. It's probable that Apple will simply update the existing iMac 5K later this year, but 8K is probably not in the cards.

Source: LG Display, via MacRumors