What makes Yishun Park a popular gathering point for Pokemon Go trainers is the four closely placed Poke Stops around the pavilion where trainers could sit comfortably to catch Pokemon. The frequent sightings of Snorlax around the area is the icing on the cake (or was this the main reason?) and adds excitement & uproar whenever it is near.
Last Sunday was my 4th trip to Yishun Park over 2 months, so on average, I have been visiting the park every fortnight. The lures were still up with some contributions from my boy and wife, but the crowd size has shrunk considerably to only about 50 players. Still a good size but no where near its former glory where you could see a few hundred players crowding the small area.
I am not sure whether the latest migration affect the spawns there, it has been spawning water type Pokemon since day one, Magikarp, Psyduck, Slowpole, and the occasional Draitini. I guess this has nothing to do with nest, but more of a natural habitat for water based Pokemon. Magikarp is good for power leveling though due to its high capture rate, and with the latest catch bonus, Yishun park will probably maintain its popularity for power players in the neighborhood.