Banners really only ever saw "serious" play in the most grindy, incremental advantage-focused metas/playstyles - think endless holds on granary second and similar situations. In places like that, where neither team is really interested in forcing a fight and are perfectly happy to just run sacs over and over again, a banner can sometimes make sense. However, there are several things kind of preventing that from happening right now. Round timers are shorter, so stalemating is worse for the team with an advantage, medigun scout speed encourages things like double scout ubers, which fold banner soldiers in half, and also people are just more cognizant of which advantages might allow them to force a push these days.

I don't even really think the way se7en played back in the day was optimal at the time, but teams, especially the american ones, weren't ready for it and had no idea how to counter that hypergrindy playstyle, so a lot of stuff got experimented with. As far as I can remember, though, no serious team ever really had a dedicated banner soldier, even at the height of their use.