Someone I know was given Austrian citizenship by marriage, and didn’t have to give up their British citizenship. How is this allowed?

This situation is known of, and was previously allowed, and the chances are that the acquaintance is of a certain age (without wishing to sound ageist probably 70 or over) Indeed the Staatsbürgerschaftsgesetz 1965 (repealed 1984/5) stated in Article 9 that a spouse could receive Austrian citizenship by marrying an Austrian (source: Stammfassung StbG 1965…/1965_250_0/1965_250_0.pdf) without any mention of the need to give up their British citizenship. This legislation was however repealed in 1984/5 and the current version of the Staatsbürgerschaftsgesetz 1985 as amended does not contain such a provision. There was only a requirement for the marriage to be intact “am Tisch und im Bett“.