Seems like the peak was around 2017 but they never performed particularly well?[1]

The problem is if the promise from the name was true, they'd be everywhere - they're not, so invariably much like vanadiun-redox or iron-flow batteries it turns out all the other details make them more expensive and less performant.

[1] https://en.m.wikipedia.org/wiki/Aquion_Energy

" if the promise from the name was true, they'd be everywhere"

Not necessarily. Lithium is still quite cheap, safety is not the number one demand - and it is mainly about optimizing production to achieve competive pricing.

So yes, mabye there are some blocking details I am not aware of, but otherwise I expect their time will come.