Reciprocal healthcare agreements don’t specifically need to be rescinded. They were part of our former EU membership and remain in place during transition as agreed in the WA. Unless a new deal is in place by the year end they will simply expire and it will be up to individual countries whether to continue with something similar, presumably in conjunction with the UK agreeing to do the same. Without an overall agreement, the existing EHIC, S1, S2 and S3 schemes won’t be available and replacements will need to be put in place. There’s no guarantee that this will happen in each country, and new schemes may not match the ones they supersede.
Personally I think there will be a deal that will allow the existing schemes to continue but there’s no guarantees.