@seriphyn Sorry for the late reply – just tested, but unfortunately is doesn’t work – for drafts or published pages… But I really appreciate you looking into this for me.
FYI – I queried the efficacy of your function with ACF support, and they said:
We’re currently looking at how we can implement validation in Gutenberg Blocks either by ourselves, or by contributing to Gutenberg as we agree we’ve had to wait too long to be able to implement this. There are many upcoming changes to Gutenberg’s saving system we hope will be hook into
The example code is too broad, and affects non-gutenberg content too, and happens too late in the chain than we’d expect, which is why we’re aiming to get a solution that works properly in the block editor – at the point the rest of page validation occurs.
