What if I microwave Beyond Meat?
There are no health concerns with microwaving food. Microwaves excite water molecules to heat food, they don't change food or make it dangerous. When a product says do not microwave it means one of 3 things:

The packaging is not meant to be microwaved: heating some types of packaging can cause bad tastes or smells in food, or cause the packaging to release unhealthy chemicals. Putting the food in a microwave safe dish solves this problem
Microwave heating causes undesirable texture changes in the food. Microwaving can sometimes cause a rubbery texture, or make it soggy
As @fraxinus rightly points out some foods can explode in the microwave,  especially food with a shell or airtight covering of some sort like eggs and potatoes. Heat causes pressure to build until the shell or covering fails, causing a 'rapid unscheduled disassembly' as they say in the aerospace industry when something blows up

Number 2 is the most likely reason the package said do not microwave.