Everyone recommends flossing – but there's hardly any proof it works

There's an inverted-U graph of approval vs. effect size. Most things with no effect aren't approved. Things with moderate effect are heavily studied until enough evidence is gathered for approval. Things with huge effect don't get much study, because it's not very exciting. For instance, bottles of vitamin C pills in the US are labeled "This product is not designed to treat or cure any disease", despite everyone knowing that they cure scurvy.

I wonder which side of the U flossing is on.

Who did ever suggest flossing to prevent cavities? That would be bananas.

As for gingivitis (and its potential to cause periodontitis, which is the real culprit) - flossing is a low-risk and low-gain recommendation, when looking at a population. On an individual level, for a patient with early periodontitis and tight-fit teeth, it can be the best recommendation ever. There is evidence suggesting that when properly applied (i.e. by a professional), flossing works great for preventing gingivitis.

Flossing is excellent though for dental professionals who otherwise would be dealing with a constant stream of heavily bleeding gums. Ugh!

Yeah but if I floss regularly my gums stop bleeding when I poke at them.

What more do I need?

Ditto parachutes, no RCTs.