I think the issue with forex is that the market is fragmented. Spot forex, futures etc.
For other markets, exchange traded and "single" markets (e.g. crude oil), one can watch e.g. COT and just miss any OTC action, which might be small in some markets so no harm done.
But in forex there is Ivol/RR, COT, FXCM SSI, Oanda'a stats, which all track some part of the activity.
Maybe some seasoned forex traders can share which are the most representative, if any.