alt Hacker Newsthe insulators on which could contain beryllium oxide
As far as I can tell, this is an urban legend. No consumer microwave oven has ever used beryllium in its magnetron insulators. Military radar ones, yes (and likely where the legend started.) Some specialist test equipment and RF transmitters too, and they all contain prominent warnings of it. Besides its toxicity, it's far more expensive than regular alumina.
That's my understanding as well, but I still wouldn't disassemble a 1960s microwave without protection (I have assisted in the dismantling of a couple microwave communications devices which did contain BeO and were also very well-labeled as such). Anything from the 80s on at least is almost certainly aluminum.