How tightly enforced are open-access embargoes?
One data point:

I am a mathematician, and I have never heard of this happening to anyone.

Many mathematicians post preprints on their website and to the arXiv, and it is somewhat common for mathematicians to also post scanned copies of the published journal versions of their older papers.

Ethical judgements may vary, but I am unaware of any authors who have suffered negative practical consequences for this, or even who have been asked to remove their papers.