This has been true for a long time in my experience, especially outside peak direction.
But it's a weird confluence of things too. For one, some fraction of fare revenue comes from monthly pass holders. Probably not as much as pre-covid, but still some fraction. And especially outside of peak direction, I doubt there is a huge quantity of money being "left on the table".
Also... like, I'm kinda sympathetic to the conductors. Either some has paid their fare already, or they haven't. Of those who haven't, some fraction will argue about it, some fraction will yell about it, and some small fraction will get belligerent about it -- which is negative for both the conductor and other passengers. And what're they gonna do? The conductor can't really throw the person off the train, and delaying the train to wait for the police to arrive is going to be popular with no one, including management.
So... I dunno. Yeah, the conductors should collect fares, and yeah, it's part of their job so they should do it. But, like most things with the T, I think there's an underlying institutional dimension here that is hard to untangle from the actions of individuals.