There should be other incentives to supporting.

Wouldn't work. People already try to strongarm the supports into doing exactly what they want under threats, at best, even in the champion select screen.

Besides, I just throw away my ban on Urgot, anyways.

Honestly I don't think that incentivising support is the right way to go. Anyone who actually wants to play support like myself would only benefit more from things like this, and thus would get an artificial ELO boost from the advantage, while it likely still wouldn't be enough to get people to want to play support that normally wouldn't. The only way to really get more people to play support is to make it feel more rewarding when you actually play it. Given that such a change would likely come with increasing their overall power, you run again into the issue of artificially boosting support mains who already do fine with the position.

The way to encourage more people play support champions is to make more interesting support champions. Characters like Bard and the re-made Taric come to mind. But currently, I think the intended support champ pool is the lowest of all the roles? Also, the community needs to be more accepting of off-meta picks for support if people enjoy supporting with them (not trying to endorse support miss fortune here). Really that goes for every position IMO. Let your teammates choose characters they find fun, they are comfortable with, and fulfill the role requirements -- even if it is not the optimum pick.