According to https://www.devin.sk/download_file_f.php?id=1191441, Devín Borough was the only area of Bratislava without official symbols approved by the heraldic commission, and the borough's government instead used an unofficial leaping bull as its symbol. This was until 6 November 2007, when the heraldic commission approved new symbols.
Of course, it also appears that the flag and its colors were based off of the unofficial coat of arms with the leaping bull (COA visible at [1]). Hence, it is assumed that the flag was also unofficially used; although, keep in mind, this is not certain, although very likely.
https://www.devin.sk/download_file_f.php?id=1191441 (flag information)https://www.fotw.info/flags/sk-ba-dv.html (flag image)
https://ives.minv.sk/heraldreg/wp_detail.aspx?ID=3264 (information on adoption of new symbols)
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