These actually still occur regularly, they're just different than they used to be.
Previously, a 4K exclusive might occur when a studio has prepped a 4K master but haven't made it available via disc or wide-release streaming (yet). This still applies to things like Die Hard sequels, National Treasure movies, etc. However, even these were subject to various qualifications as when they were finally released there were often further improvements, updated color grading, etc. that never made their way back to K (Taxi Driver, Lawrence of Arabia, Mask of Zorro, etc.).
Nowadays, with the explosion of boutique labels, the number of 4K disc exclusives has skyrocketed. However, that doesn't mean K isn't getting exclusives anymore. K is still leading in disc-quality exclusives for direct-to-video offerings. Most of these types of titles don't have disc releases planned, and will only get them if a movie becomes a hit on streaming. Recent examples: The Surfer, Shadow Force, On a Wing and a Prayer, The Underdogs, and On Swift Horses.
Sure, the argument could be made that the 4K disc exclusives are sought after items while the disc-less direct-to-video titles aren't. Fair. But, K is still the exclusive source for the highest quality audio/video of hundreds of films that grows weekly.