Plan S, an initiative that aims rapidly to accelerate the transition to open access scientific publishing,
was launched by Science Europe in September 2018.

The central tenet of Plan S is that:  “After 1 January 2020 scientific publications on the results from research funded by public grants provided by national and European research councils and funding bodies, must be published in compliant Open Access Journals or on compliant Open Access Platforms.”  The Plan states that hybrid journals,  i.e. subscription-based journals which publish individual articles on an open access basis, will not be compliant and it requires publishers to enter into transformative agreements under which they will commit to convert their subscription journals to open access within a short timeframe.

https://beta.iop.org/iop-responds-to-debate-over-plan-s