With immediate effect, visas issued to family members of British citizens on the basis of the right of freedom of movement will be given a validity until March 29th, 2019.
In the case of a trip designated for a longer period than March 29th, 2019, it is possible to apply for a visa under the general requirements for visa applications according to the Schengen Visa Codex or the rules of the Residence Act instead of applying for a visa in accordance with the right of movement of family members of British citizens.
After March 29th, 2019, visas for an unlimited period will not necessarily entitle their holder to enter and stay in the Schengen territory or Germany any more. In the case of a no-deal-Brexit British citizens will no longer enjoy freedom of movement as from March 30th, 2019. Therefore, visas which have been issued to a family member of a British citizen on the basis of his/her freedom of movement within the EU cannot be used any more.
For more information on this subject in German and English language please refer to the website of the German representations in the UK ( http://uk.diplo.de ).