Not wishing to bang our own drum too much, but it seems we were prescient about the difficulties of introducing double volume caps on dark pools. See: MiFID 2 double volume caps - the end of dark trading?
The European Securities and Markets Authority (ESMA) has decided to delay publishing double volume cap data on dark pools. The data was intended to determine which stocks had breached dark pools trading limits and would therefore be subject to curbs.
ESMA said in a statement: "The current quality and completeness of the data does not allow for a sufficiently meaningful and comprehensive publication of double volume cap calculations as required under MiFID 2/MiFIR. The publication would have resulted in a biased picture covering only a very limited number of instruments and markets."
In order to avoid the creation of an "unlevel playing field" implementation will be delayed until March.