EDIT 5/5/2019 5:20pm PT: Added further roadmap illustrations to clarify the disparity. We've contacted AMD for comment and will update as necessary.
Ryzen 3000 has been one of AMD's most anticipated products. AMD's new chips are expected to launch by the third quarter of the year, or perhaps late in the second quarter (around Computex). But we aren't certain about the release for the high end desktop Threadripper processors because AMD has removed these CPUs from its roadmap that it shared during the company's Q1 earnings report. Where Ryzen 3000 and Threadripper 3000 previously stood alongside each other, there is now only Ryzen 3000 with the statement "mid-year."
This is somewhat confusing and has sparked debate because AMD made the change very quietly and without comment. The initial reaction to this sort of news might be that there's some sort of issue with Ryzen, but that's highly unlikely considering that AM4-based 500-series motherboards are expected to debut at Computex and AMD has also already shown a demo of an eight-core Ryzen 3000 CPU. Threadripper 3000 will likely be similar in design to the EPYC Rome data center processors, and AMD has already confirmed that Rome is working as planned and will launch in Q2 in limited quantities (high volume manufacturing in Q3).
So, what's going on with third-gen Threadripper? The changed listing could just be an unintentional removal, and Threadripper will arrive as planned this year, but it could boil down to a few issues (or a mix of them).
Threadripper, Ryzen, and Epyc Rome use the same 7nm compute chiplets, meaning these chips will compete with each other for the best dies, of which there likely aren't many because the 7nm node is quite new. The newer the node, the more defects, meaning more low-quality or defective dies. Rome will, of course, get the best of the bunch, while Threadripper will get the chips that clock high at decent voltages, leaving Ryzen with the worst chips. However, as Threadripper is a niche product and supply could be tight, there might not be any room left for Threadripper to snag some of the best chips that would make more money if AMD sold them as server CPUs.
Motherboards are also a complicating factor. TR4, the Threadripper socket, hasn't seen an update since 2017 and still uses the X399 chipset. Granted, X399 boards can be quite good, but AMD will likely want to update it for Threadripper 3000. While we’ve seen leaks and news about 500-series boards for Ryzen, there hasn't been any news of a new chipset for Threadripper, making it unlikely the new Threadripper chips are on the near horizon. We may just need to wait for the next horizon for Threadripper 3000.