Raspberry Pi Foundation failing to communicate their development of this stepping will results in tens of thousands recently bought A2-stepping devices ending up in the landfill.
I think they didn't preannounce because it could have taken longer than expected and people would have held orders in the meantime, which could have been a problem. [1]
Although they did do this a little bit:
> Eventually all Pico 2 products will use the A4 stepping. While the A3 stepping will not be made available to silicon customers, approximately 30,000 units of A3 inventory will be used to build Pico 2 and Pico 2 W products.
~~I'm hesistant to buy a Pico 2 or Pico 2 W right now because I might get this weird stepping that could have issues almost no one else ever sees.~~ [edit: not so much after seeing phire's comment and the bootrom-only changes in the datasheet.]
All the hardware changes and most of the bootrom changes went into A3. A4 is only a minor bootrom update.
The chance of an A3-only errata is tiny.