To my knowledge there is only one implementation of R7RS with a low-level macro system which strongly enforces phasing so far: Gerbil.
Larceny requires explicit phasing for sure, and I think Loko does as well. Racket is "hyperexplicit": in R6RS explicit, an identifier is either known at a certain phase or it isn't, whereas in Racket an identifier can also have different definitions at different phases.
But also I sense this is a touchy issue for those who reject R6RS, and SRFI 211 is thus far a very politically neutral document in that sense.
Only to some extent. Implicit phasing was designed by some of the R6RS editors and implementers, and most R6RS systems use it; per contra, Chicken is quite anti-R6RS and yet uses a system of two explicit phases: one for phase 0 procedures and all macros, and the other for all positive-phase procedures.
Daphne