It still seems to work here with photoshop cc 2017.0.1 and mac os x 10.11.6
Is there a particular tool you get that message with when you try to set the opacity to 100% by pressing the 0 (zero) key?
It does work when you just press 0 (or 1 for 10% and so on) and you are using a brush or a similar took, but I guess when you select the tool opacity % area you can't input 0%. Perhaps it makes sense for those that may be inputing 0 for some reason and expecting some kind of result. You get that window message and then it turns to 1%. It's no biggie I think I thought the other option stopped working for a second. It's late and I do a lot of things from instinct now and just just threw me off for a bit. Nothing has changed.
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You must never select the flow or opacity fields when inputting by shortcut.
Click on a blank part of the option bar to deselect the opacity/flow fields.
Then 0 for 100% Opacity, Shift 0 for 100% Flow.