According to the
Lost Era novels, the ban on women serving on the High Council was instituted after Azetbur's death as a reaction to her rule, to prevent a woman from becoming chancellor again.
Although TNG is inconsistent about whether women can serve on the Council; in "Reunion," Gowron offers K'Ehleyr a seat on the Council if she'll back his claim to leadership. The ban was only introduced later in "Redemption."