I mean, I can't source it other than saying you gotta talk to minor leaguers / MLB wash-outs. Fans like to focus on how much money Juan Soto or Aaron Judge is making and what goes unreported / unnoticed is how many journeyman making the league minimum get fucked by the organization the moment they get injured trying to make prime time.
But I can point to the fact that MLB changed eligibility rules that when you demote someone to AAA, they have to spend a minimum amount of time there. Because prior to that it was a game of: Have minor league options? Great, pitch in NY on Monday, hop on a plane after the game and be in Las Vegas for the minor leagues on Tuesday. Oh you had your minimum 2 days rest? Get back on a plane and meet us Atlanta to pitch on Thursday. Now back to Las Vegas on Friday. And they don't get paid MLB salary for the days they spend in the MiL.
Now you'd think someone said wait a sec, that's fucked up, but the reason the rule changed was that veteran relievers were complaining to the MLBPA that the lack of MiL options was destroying their contract opportunities in free agency. They 'paid their dues' only to be told to get fucked, the inability to demote you to AAA at will is too constrictive.
Not to mention the fake IL assignments, for which a GM, Billy Eppler, was fined by the league (and resigned his position).
And then if they actually get hurt to the point they can't pitch anymore, they gotta lawyer up to get their benefits paid out.
Every reliever in MLB is someone who wasn't good enough to be a starting pitcher. So they get converted. At that point they are practically expendable bodies except for the rare few that ascent to Aroldis Chapman status. The relievers all know that no one gives a shit about their well-being, they just hope they can last long enough for a decent payday. It's the dirty underbelly of the MLB, and no one is going to stand up for the guy who comes in to throw 15 pitches in the 6th inning, consisting of 8 fastballs and 7 sliders, when the team is down 6-1.
There's a reason Seth Lugo fought tooth-and-nail to be a starting pitcher. Not only is the pay a LOT better, the organization treats him like a human being who has a valuable skill and believes in things like 'rest'.
Also look at how fans and pundits talk about 'the bullpen.'