This is an old hat. Please read some very plausible reasons here (in the end, we don’t know).
You know SketchUp from time to time updates client system requirements (higher Visual C++ library or OpenGL version to take advantage of improvements). If you put yourself into the position of the opposite side, of developers, you would see that there are also development tools, requirements and infrastructure. I speculate these equally need to be updated sometimes. A hint is the addition (not replacement) of a web-based version which – this is totally new – has no public version information and is continuously updated (continuous delivery). Besides knocking out piracy, this lets free users test the stability of the application, long before a Pro release is approaching (and needs to be stable).
Don’t you think it would be rather late to announce if there won’t be such a version (SketchUp 2019 Pro desktop)? A release takes a year in the making, so a decision to make no release would be fixed early in the planning. If you read between the lines (you can not read all lines, but luckily the ensemble of the SketchUp community can), you could find:
I agree, an official communication (that’s not thomthom’s job) has been missing. But if you plan to make a surprise to your children, and it delays by a day (and then just only one day more, and then another day more), would you be going to spoil the surprise by telling them it is just delayed a little?