For me, these kind of comments are more about what you might want to buy or not buy, as opposed to wear or not wear if you already own it. E.g. if I'm someone who cares about being very up-to-date, and I don't have strong feelings one way or the other about Birkenstocks, I might wait and see what the hottest trends in sandals are this spring before I buy a new pair. But if I already own a pair and like them, I'd go ahead and wear them. Same with plaid shirts.
I was at the thrift store yesterday, and I saw several of those poly knit tops with an empire waistline, in prints. Remember those? That's the kind of thing that I think of as being "truly out" at the moment. (Not that you couldn't make them work if they are an integral part of your style. If you had a boho Stevie Nicks thing going on they could work.) Plaid and Birks have barely gone from "super-trendy" to "a bit mainstream" at this point. It's kind of a sine curve or a spiral - things don't go from instantly in to instantly out anymore, they rise and fall in popularity (and have niche popularity separate from that) over years and decades, not a season or two.
Last summer I had a pair of cheap Birk knockoffs; they got worn a lot and then thrown away at the end of the season because they were already falling apart. I haven't decided whether I'm going to try to get a new pair or not. I will probably let fate (in the form of a sale) decide one way or the other.
Plaid, on the other hand, has always been in my wardrobe and always will be. I think saying plaid is 'out' is like saying stripes or out, or a certain color. It's more about what's new, not what's out, you know? Pantone doesn't say to everyone "here's the color of the year, if you ever wear anything other than marsala you are a fashion failure", because that wouldn't make any sense. Same with classic features like plaid or preppy or military influence or a certain kind of fabric or seaming. If they resonate with your own aesthetic preferences, they can always be worked into a look, whatever's trending.
