Date codes are often "year, last-2" followed by "week, 1-52". So in
this example, late August of 2020. Coming up on 3 years in storage.
So a question (if not "the" question of adherence to spec conditions)
might be the conditions of storage. Like, factory-bagged in dry nitrogen
or vacuum is not going to "consume" storage life (by corrosion processes)
the same as an open waffle pack in a desk drawer.
Now you could ask yourself (or better yet, someone who knows) whether
18 months is the supplier's or the aassembler's "rule", whether your PO
to them references any such rule or it's being thrown at you without any
such call-out, whether there is any actual material quality concern (IQC
says what?) or whether they just want to get paid for shopping some more.