The iFrame section is a sunset section and is no longer available in the add section menu.
It has been replaced with the more up to date HTML section which can do everything and more than the iFrame section used to do.
If you have published Apps with iFrame sections in them they will continue to work. If you need to update them you can but you may get improved functionality if you replace them instead with the newer HTML section.
to find out more about HTML sections click below:
The HTML Section