The laws are set, however the policies and guidance are still being finalized at the IRS (as of mid-December, 2018). You can observe this yourself by searching the web for "2018 IRS Form 1040" and etc, many of the forms returned are still listed in 'draft' state, and bear the warning "DRAFT AS OF {DATE}, DO NOT FILE". While this might seem unusual, even in a normal year the documents are still being revised and updated until late into the end of December. The IRS has a decent amount of leeway, because the official deadlines for banks and businesses to provide their customers with official documentation (such as W2s, 1099s, etc) are still over a month away in most instances.