The thing is, this ban was introduced at a time when games were typically played for about 2 months, and the Grand Order (GO) wasn't usually a factor before 4 weeks. Players focused on developing their home castle and building it up, and losing it early on meant surrendering. That's why encountering Solmyr, Dimera, and similar heroes was considered overpowered in the early game (around week 2). Nowadays, by week 2, players are already pushing for maximum stats, so having a few angels in their treasure (although engaging in a battle with a bolt for 200 gold using only angels is still risky if the rest of the army has fallen in previous encounters) allows them to leave the mage to roam around the map and guard castles, but by week 3, he'll already be an underdog.
Therefore, such a ban is outdated. Mages aren't played much anyway, as players prefer to choose a warrior. The presence of the red orb, combined with the above, gives mages a chance, but only in the early game (around week 2), and even then, only if the opponent is unlucky with their spawn points. With these rules, the mage's role has been reduced to a niche strategy :D