It is possible for a British citizen to fulfil the residence criteria under the Withdrawal AgreementWithdrawal Agreement The Withdrawal Agreement sets out the terms of the UK’s withdrawal from the EU and provides for a deal on citizens’ rights. It sets out a transition period which lasts until 31 December 2020. During this time you can continue to live, work and study in the EU broadly as you did before 31 January 2020. If you are resident in Austria at the end of the transition period, you will be covered by the Withdrawal Agreement, and your rights will be protected for as long as you remain resident in Austria. Any rights that are not covered by the Withdrawal Agreement will be the subject of future negotiations. https://www.gov.uk/government/publications/new-withdrawal-agreement-and-political-declaration in more than one EU country (for example by having gained permanent residence in one EU country after more than five years working there, and then subsequently moving to work and reside in another EU country prior to the end of the transition periodTransition Period The transition period (also referred to as the implementation period) is the period following the UK's departure from the European Union (on 31.01.2020) until the end of 2020. An option to extend this period has not been taken up by the UK government., you moved to work and reside in another EU country).
If you have gained a permanent residence in one EU country, you will be covered by the Withdrawal Agreement if you return there within five years of your departure (although this will depend on the host country rules as to whether you need to apply for a new permanent residence status under the Withdrawal Agreement). You can apply for protection of your Withdrawal Agreement rights in another EU country while still retaining the rights to permanent residence in a country you lived before for a certain period.