The big Hero Pool experiment for Overwatch
is coming to an end, with Blizzard
announcing - "We’ve found that the introduction of Experimental Card and increased hero balance updates has helped us work towards a healthy, changing meta in Competitive Play without needing to disable heroes".
As per above the whole Hero Pool deal was essentially a roster that would block out certain heroes for competitive play - a measure taken to diversify the overall meta and team diversity. The reception and overall implementation of Hero Pools was met with some backlash but the goal was something people have been looking at Overwatch for sometime. That is balancing and team composition for competitive play.
Currently there are no plans to bring Hero Pools back, and as for the Overwatch League - "Hero pools in OWL will now last two weeks before rotating, and after each two-week hero pool period, the next two weeks, including the tournament matches, will be played with no hero pool in effect." Which sounds a little confusing but the take-away is that competitive Overwatch won't be placing restrictions on team composition.