According to Ryanair's website:
Irrespective of a passenger's visa requirements, all non-EU/EEA citizens must have their travel documents checked and stamped at the Ryanair Visa/Document Check Desk before going through airport security. Your boarding pass will display this information for routes that you must comply with this Visa/Document checks requirement.
Does this mean British citizens will need to visit this special visa/document check desk starting from Friday, January 1st? There is a similar requirement on the WizzAir website that doesn't seem to have an exception for British citizens.
Update: this isn't about Schengen immigration formalities. This is purely about Ryanair's own quirky policy. See How to fly with Ryanair as a non-EU citizen?