When applied to still alcoholic beverages, does the term 'flat' mean "lacking taste" or "lacking alcohol"? It can mean "not fizzy anymore" but only if the beverage is supposed to be so, like champagne. What if someone uses the word to describe, for example, beer which, as far as I'm concerned, is not considered an effervescent drink despite containing bubbles in it?
Beer is an effervescent drink because it contains bubbles in it. If the beer is flat, that means the bubbles have gone from it. Some people might describe a beer as flat, meaning that it has less bubbles they expect.
Americans often find English ales to be warm and flat. Certainly, a cask-conditioned ale is less "fizzy" than a keg lager, but it should never be flat and should always support a good head.
It would be unusual to describe a still wine (for example) as "flat". It could mean "bland", in a figurative way.
The 2016 vintage zinged with bright fruit flavours. By comparison the 2017 vintage is flat, lifeless and uninteresting: a great disappointment.
It couldn't mean "lacking alcohol" not least because modern production techniques mean that the alcohol level is completely controlled by the producer.