After the 31st of December 2020 the Brexit will definitely enter into force and UK citizens won’t be considered EU citizens anymore.
However if you are an Italian citizen who in the past naturalised UK, you can request the reacquisition of your Italian citizenship, which you lost before the 1992 and do not lose the right to be an European citizen.
In fact with the introduction of law n. 91/92, obtaining another citizenship does not imply losing Italian one.
1) Recovering the right to Italian citizenship lost because of naturalisation before1992
Italian nationals born in Italy who naturalised British before 16th August 1992 lost the Italian citizenship. It is possible to reacquire the Italian citizenship, and still maintain the British citizenship, by transferring the legal residency in Italy.
They may reacquire Italian citizenship:
1) Automatically after having transferred their residence formally in Italy for one year, or;
2) By signing a declaration in compliance with art.13, paragraph 1, lett. C Law 5th of February n.91 1992, at the Italian Consulate and taking up residence in Italy no later than one year from the date of the declaration.
In Italy, applicants will have to apply for a residence permit specifically for the purpose of reacquiring of Italian citizenship at the relevant Questura. Once the above-mentioned permit has been issued, applicants will be able to apply to be registered with the Municipality of choice.
2) Recovering the right to Italian citizenship lost after marrying a UK citizen prior to 01.01.1949 (British nationality and status of aliens act, 1914)
Italian women who, by virtue of a marriage to a British national before 01.01.1949 were automatically and unintentionally naturalised British, may apply to the Italian Consulate for recognition of uninterrupted possession of Italian citizenship in compliance with artt. 46, 47 DPR 445/2000, by signing an application and submitting the necessary supporting documents.
However, women who married after 01.01.1949 and intentionally acquired British citizenship have lost the Italian citizenship.