IPC doesn't care about silkscreen; 2221A says to reserve space for designators and not to hide information under components, but doesn't make any statements about what a component should look like.
It's perfectly producible to leave minimal silk. 80s Tektronix had hardly any: rare if any designators even, mostly a plus to indicate polarized capacitors. Modern high density builds don't afford much space to silkscreen, for obvious reasons.
Personally, I use a circle with a plus to one side and a minus on the other (actually an arc segment, mimicking the polarity mark on the sleeve). The half-shaded style is fine too, and there's a concentric arcs style I think that's more European in origin.
I don't like the half-solid style, the white balance is just so awful. But as you can see, that's an aesthetic statement, nothing more technical than that.
Tim