Are you saying to leave the expired license installed, but simply disable the Authorization Rules that apply to Plus licensing?
Interesting question. What I can tell you is that if you were to remove the expired Plus License, then your ISE Admin menus will reflect that (you won't have access to stuff that relates to Plus Licenses). So perhaps don't delete the license. But you will probably get constant Alarms until the license is removed.
I would like to hear the outcome of this question because I have also wondered how to approach this. Of course the obvious approach is to buy new license, but it is valid to take a "payment holiday" ;-) and press the Pause button on those polices. Perhaps in future you can resume those Policies when you have budget. But the interesting question is what the impact is of leaving the config in place and just disabling those policies so that they don't get hit and count towards a Plus License.
If you leave an expired Plus license installed, I wonder whether you will get a License Warning, or a License Violation. I think it's a Warning. And you can probably live with that.
