https://www.beyondtheboxscore.com/2015/5/19/8614817/lineup-protection-myth-reality-exists
https://blogs.fangraphs.com/players-view-does-lineup-protection-exist/
There's a couple of articles on it. I like this quote - "Lineup protection may or may not exist. Studies suggest it doesn’t, at least not statistically, but many within the game insist it does." I should trust that it exists because pitchers say it matters to them. However, the statistics suggest it doesn't actually have a major impact. Perhaps perception isn't reality. I don't think anyone should be heavy on one side of this discussion.
Suggesting "of course it does - these pitchers say it does, so it must" doesn't mean a lot to me as a lot of people also tell me that God exists.