I gave some remarks on the code, which were processed.
If there is a feeling that adding the HeatIndex complicates the example, maybe this example should be a separate file, named "DhtHeatIndexSensor"?

OK, I understand.
My confusion was caused by the second sentence at https://www.mysensors.org/hardware/micro : "... it operates at 1Mhz ..."
That's a mistake then?

Under "Signing CLA", the link to clahub.com is since long dead.
Maybe you can follow this example: https://github.com/chocolatey/choco/issues/2036
and replace with https://cla-assistant.io/ ?

I also have a few of such Arduino Unos with a 3.3V selection switch from the shop "Open Smart" on Aliexpress.
See picture at: https://michiel.vanderwulp.be/domotica/Modules/ArduinoUno/#open-smart-3-3v-5v-switchable-uno-specification-1
They work fine, also on 3.3V.

But indeed, running it at 16MHz on 3.3V is out of spec...

I even managed to solder a nRF24L01+ to the board so everything works on 3.3Volt, and this node works fine.
This took a lot of scratching copper from the board...