Sorry mate, but that is just bollocks and insulting to boot.
He's not pulling out of 50/50 challenges at all, and certainly not because he thinks he's leaving. It's laughable. Is nobody allowed to tackle him in training either?
He's severely lacking in confidence in the final third, and his decision making is poor, but to suggest he's not trying is not on, and frankly is insulting the intelligence of Pep and his team.
In your defence, it's a criticism I see pretty often, but it's a lazy and outdated one.
Every time a player avoids a heavy challenge or doesn't chase a lost cause, he's a shithouse. But that's not how it works. This isn't the 20th century.
Sterling is probably above average among wingers for his defensive contribution and physicality. He won a throw in last night by out muscling a much bigger player off the top of my head.
I appreciate that you've heard he's on the way out, and I think you're probably watching the game from that perspective, looking to confirm your suspicions. But it's not fair to accuse him of something without any basis whatsoever.