As an editor with MuddyUm, I ding any writer who puts a being verb in the first sentence. But I only ding Is, Are, Was, and Were. Most of those other ones on that list are OK.
Lots of writers start out with "X is Y." Boring! In most cases the sentence immediately following makes a better opener, with minimal to no reworking. I suggest they ditch the first sentence and start with the second. If it needs reworking to make sense after deleting the first I suggest something. I'm nice that way.