Here is an article that definitively says lineup protection exists, complete with numerous quotes from many recently retired players. (It’s a few years old.)
Of course, it concludes that lineup protection leads to an uptick in walks, which is exactly what not protecting your best hitter also leads to (if he’s patient). So their claim that it exists proves it doesn’t.
And in this age of analytics, where many players have their weaknesses mapped out and where OBP is valued more than it used to be, the notion of walking hitters rather than attacking their weaknesses seems far less likely…