Attached patch makes player unable to start researching new future tech when there's real tech available. Already ongoing research is not affected. This is consistent with other loss of tech requirements. I suspect that we have to change that in general (current code might allow holes to appear in the tech tree even when rules deny them), but that's for another ticket.
Split from anonymously reported #44419.
When a player is already researching future techs, they can always continue researching more of them, even if they have lost some real techs so that starting future tech research should be impossible now.