I just played a match of battlegrounds and it didn’t include Beast - type of minions, but when I got a triple on tier 5, there was a Megasaur to pick as reward. Took a screenshot of it but dunno how to post it.
P.S
Hoping to see the day when it gets removed from the pool entirely 
Megasaur is part of the murloc pool, which makes sense.
That’s true, but I think Blizzard would’ve made a reworked version of it like they did on lower tier Menagerie-buffing cards if they intended Megasaur to be part of Murloc-pool
Could be done to make it more logical to follow the battleground rules when beast are not in play.
But the card is also available in the normal collection.
maybe they considered it but chose the less amount of work option ? 
To me it’s fine as it is. The tribe ban is for cards related to that tribe, non necessarily every card in that category, so it makes sense that Megasaur having Murloc synergy is still in the game.
One thing that could be proposed to Blizzard, since they did remove/change some cards, is to substitute all these cards from one tribe that benefits others and put in something neutral or same tribe. As you said, they did it for the menagerie, they could do it for this one too.