Seems to me that Redshirting is typically done by decision of the athletic department, not the players themselves.
I know that certain professions allow for sabbaticals, typically without pay, meaning you take a leave of absence while still maintaining your position.
It's one of those situations that no one really thought of before NIL, and now that we have it, it's a problem.
Personally, when NIL came down the pipe, I thought to myself "We will see similar tactics that NFL players have used, such as holding out."
Personally, if you sign a contract with a company, and if it wasn't explicitly stated in the contract you had control over taking a redshirt due to lack of playing time, you probably don't have a right to it. Redshirting, excluding players applying for a 'medical redshirt' (which is different), I think the power of Redshirting has always been the responsibility of the athletic department/football team.