Hm, not working for me in 60.0.3112.101 either, disabling chrome://flags/#browser-side-navigation didn't help either so probably unrelated to  issue 747812  and PlzNavigate.

Navigating a tab from the omnibox to 'data:text/html,TEST' does work so there's something special about window.open

This is probably fallout from  Issue 594215  - Disable page-initiated main frame navigations to data URLs

It's not clear to me whether this should apply to the console.

To be clear, I only used the console as a simple example. This was actually being used, and is now broken, in a an application where an HTML report is being generated in a new page, to allow for easy printing. Is blocking this ability a new feature rather than a bug? Is there a workaround?

Yes, this is intended. It applies to all data URIs. Please see https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/GbVcuwg_QjM for the rationale.

We want data URI behavior to be consistent with other non-web URLs such as chrome://, which is why console is also blocked. E.g. typing this in the console also opens a blank window:
window.open("chrome://settings");

> Is there a workaround?

Yes, please see https://bugs.chromium.org/p/chromium/issues/detail?id=594215#c64

OK, thanks.

Thanks, merging this into  Issue 594215 .