There's sort of a "legal void" when it comes to index options. Previous to the "Legends" rules, there was a chart that specified how you used models with gear options not in the codex, which basically stated that you used the updated datasheet (the codex's one in this case) but could use the old wargear paying the price listed in the index.
Most units in this situation got moved to "legends", making them illegal in most tournaments, but not the GK librarian with shield, because it was never a model released by GW. In fact, the only reason the GK librarian had the shield option is because it used the SM librarian datasheet. The storm shield option on the SM librarian has become a Legends option, making it clear that it is no longer tournament legal.
Legends were supposed to solve all the index/codex problems, so there's no longer an official way to use index options, nor a legends clarificiation, so wether you can use librarians with shields is up in the air. Most tournaments just ban legends and index options, so don't get used to those shields if you plan to play competitively.
By the way, you can legally have a 2++ save with the current rules. A brother captain or grand master with sanctuary or heed the prognosticars and a stave still has a 2++ in combat (a 3++ with a +1 to the roll, technically).