Is it muttar to have a beyond meat “burger” on the nine days (is it an issue of maris ayin etc.)?
The rule with marius ayin is that it only applies if the permitted version is not so common, however if it a common occurrence, it doesn’t “look bad” because people will automatically assume that he is eating the permitted type. For example, nowadays a lot of ice cream is pareve, therefore we can serve pareve ice cream at a meat meal, because people will not assume that it is milky. Regarding eating pareve burgers, it depends on the local habits, and if there is a significant amount of people that are eating vegetarian and similar types of burgers. In Eretz Yisroel most burgers may in fact be soy or vegetarian, in the United States it depends how “in” it is to eat this type of burger. Even if it is not the norm to eat such burgers, In the worst event, place some of the wrapper on the table so people will be able to see what it really is.
Pleisi 87-8, Mishne Halachos 5-96, Poskim.