tax rate means nothing, the players get paid a set NET RATE by the teams, when the tax rate goes to 50% the players get the same NET pay, the club just pays them more...
That sounds incredibly unlikely to me; any evidence to back it up?
Never heard of any company or organisation ever doing such a thing. Individual contracts may be agreed taking into account the rates of the time but (for example) in 1974 the top rate went up to 83%. Any club/company contractually obliged to guarantee a net income under those circumstances would very quickly go out of business.
Any club who work like that would be barking mad in the current financial situation.