It only became so in legal writings after 2010
Until recently, law schools taught attorneys that "shall" means "must." That's why many attorneys and executives think "shall" means "must." It's not their fault. The Federal Plain Writing Act and the Federal Plain Language Guidelines only appeared in 2010. And the fact is, even though "must" has come to be the only clear, valid way to express "mandatory," most parts of the Code of Federal Regulations (CFRs) that govern federal departments still use the word "shall" for that purpose.
Well, according to these Trump ass kissers........when the writers of the law stated.
".....the Internal Revenue Service
“shall furnish” ..........they really meant that it depended on:
Whether the requested tax returns should come from ANYONE but an orange clown
Whether the tax returns were requested on a Tuesday
Whether the requested tax returns "disturbed" the delicate sensibilities of Trump ass kissers......
etc..........I am sure that the writers of that statute were a bit "confused"....LMAO