jgsugden
Legend
In some campaigns that may be true. In most, such as those that follow adventure paths ....My experience as well. The vast majority of opponents are humanoid, heat metal was used frequently...
Talk a look at Lost Mine of Phandelver and look at the chapter where the PCs will attain level 3.
Wandering monsters? 1 of 8 options have metal armor.
Fixed encounters? There are 5 separate locations with some of them having multiple encounters within them. 4 of them have no foes in metal armor in any of their encounters. In the 5th there are 10 encounters and only 3 have metal armor. A few more have shields that can be doffed in a round.
The next chapter when they're level 4? There are no encounters with any metal armors, anywhere.