In prior years RECF has confirmed that the plastics used for field elements are legal plastics (cf. this post, for example: What happens to Game and Field element kits after season - #6 by Sidoti). Common sense: there is a reason certain plastics, like plexiglass, are illegal, so why would a shattering plastic be used for field elements?
Historically, we saw a lot of turning point flags as a source of plastic for the next season (tower takeover). Of course, it is not the fact of being a field element that makes it legal (or illegal), but rather that the material is of a legal type and thickness per rule R18…(occasionally some people become confused by the “red box” below rule R6h about using field elements on robots).