This seems to be a proposal from the ECA, which aims to solidify the positions of the clubs in the CL. In theory, it would protect teams like Ajax, Dortmund, Lyon, and Monaco from losing out on their best players whilst they're in the CL. On the other hand, it would also protect teams like Barcelona from losing out on players to the likes of PSG and City. This would also raise the prices of non-CL players as the demand for them would increase.