There are many applications for temporary portal-drop restrictions, as portal dropping can be disruptive: portals allowed in game lobby only, during events, to prevent path-blocking or using portals to visually obscure things, etc. Currently the only toggle available is during world upload.
At the moment, Udon can restrict portal drops by placing a spawn transform at the player position. (I was almost ready to release a toggle prefab that uses this method.)
With the upcoming portal placement feature, this workaround will likely no longer be possible.
Please make the "allow portals" to be an udon-exposed feature.