It was explained for example in this thread.

Later, other changes were made, so at the moment the difference in content using the old prompts has diminished, although it remains noticeable, but especially, as I already told you, now the definition is gone, the creations seem most of the time approximate and blurred compared to before.

I just would like to know if this is final (i.e. quality was intentionally lowered to limit the consumption of computational power, or to select a specific audience, etc.), or temporary (i.e. quality was not intentionally lowered, or was intentionally lowered for test purposes, etc.), and it will be fixed.