The reason why you can't put bread in the microwave is because microwaving bread can lead to a chewy, rubbery texture and uneven heating of the bread.
Putting bread in the microwave can also make the bread turn out hard as well if you microwave the bread too long.
However I will sometimes microwave some sliced bread by breaking it up in pieces and adding butter to it to melt onto the bread.
I've never had any issue microwaving the bread and butter for around 60 seconds to a little over a minute.
Just keep an eye on the bread if you're microwaving it to ensure it does not dry out or overcook.
Microwaving heats the bread from the inside out so you miss out on the nice crunchy exterior you could get from toasting the bread.
Microwaving bread also gelatinizes the dough's starch and gives it an unpleasant soft and chewy texture.
Bread is better heated in the oven or in a toaster oven but if you do want to microwave bread only do so for a short time.
You can also add a glass of water to the microwave to add moisture to the bread when microwaving it to help prevent it from drying out.
If you microwave bread, microwave the bread in 30 second to 40 second intervals and check the bread to ensure it's not drying out.