The exploit for getting unlimited emeralds from librarians with bookshelves was fixed, but another exploit was missed. 

You can buy 4 glass with 1 emerald from a librarian, and if you cure a cartographer that's leveled up enough to buy glass panes for emeralds, he can buy barely any glass pains for emeralds, getting the player a source of infinite emeralds. (picture below)

This really defeats the purpose of players building farms and gathering resources to get emeralds, and I believe it should be fixed because it's the same type of issue as the unlimited emeralds bookshelf librarian exploit, which was fixed in 1.15.