The Verdict: Need More Info
It turns on whether your citizenship ended by release, by renunciation, or not at all. A determination of citizenship at a Croatian embassy, consulate or police administration will establish which.
The three lead to very different places. Release (otpust) leaves several routes open. Renunciation (odricanje) as an adult closes the door for good under article 21(2). If neither happened, you are still a citizen and simply undocumented.
The Ministry of the Interior holds the registers of both release and renunciation, so the answer exists on file even where family memory is vague. Old papers help: a surrendered Yugoslav passport, a decision handed to you in the 1960s or 1970s, letters from a consulate.
A rule of thumb while you look: release was something people asked for so another country would naturalise them. Renunciation was a deliberate declaration by an adult who already held another citizenship.