Time is a flat circle, and has new content. Developer Cyan Worlds had added another area to its modern remake of the classic 1993 adventure game. The expands the game with a "snowy, mountainous seascape", which becomes available to players after completing the main story of Myst, bringing "all new narrative beats, new lore, and new sights to behold".
We should pause a moment, however, to clarify what "new" means here. The world of Rime is indeed new to Cyan's contemporary overhaul of Myst, but the history of this island stretches 
back a quarter of a century. While Rime never appeared in the 1993 edition of Myst, it was introduced in 2000's , which converted the game from pre-rendered images into full realtime 3D.
Rime is more like a bonus area than an expansion, a reward for completing the game that provides some additional context for Myst's story. As in the [[link]] original version, the rebooted Rime centres around a laboratory space dedicated to research of the game's Linking Books. As you'd expect from a remake, though, Cyan appears to have given Rime a lick of paint, stating in the update's Steam post that "sweeping vistas, beautiful ice structures, and fantastical natural phenomena await new [[link]] visitors".
As for why Cyan has added it into their modern remake, it appears the community has been requesting it, with the developer proudly proclaiming, "You’ve been asking, and we’ve been listening." Alongside adding Rime, the update makes several smaller additions to Myst, updating DLSS support to version 4.0, adding a zoom function, and making some "visual improvements" to the Myst island library.
Designed as a hybrid VR and flatscreen title, the Myst remake released back in 2021. Chris it at the time, finding it to be a faithful redux of 
the original, albeit perhaps a little too faithful. "It is 
nice to be able to actually walk through the world this time, and for a while, it's novel experiencing a familiar place in a new way," he wrote. "But even in the 3D, the classic puzzle slideshow still feels like a series of pretty pictures to be looked at rather than a real location to visit."