Most of Y-wings were from Gold squadron, primary mission was to attack Shield Gate
In the Battle of Scarif only X-wings (12) and one U-wing made it to the surface to help ground forces. No Y-wings as we can see, presumably because they were to slow to attempt this (even some X-wings didn't make it).
Shield Gate was most important objective for the rest of the Rebel fleet, because with the shield up members of Rogue One could not transmit Death Star's plans even if they got them. Secondary target was small Imperial fleet above Scariff (2 SD and TIE's ) . Y-wings engaged Shield Gate, losing some in the process, but they really could not do much damage to Shield Gate . Instead, suicide attack with Lightmaker was used.
Overall, most of Y-wings in the battle were from Gold squadron which was dedicated Y-wing unit. Blue squadron was mixed unit, but X-wings made majority of their fighters. I assume few Y-wings they had joined Gold squadron in their attacks, since majority of their unit was on the surface and they were separated from them.