The Tax Cuts and Jobs Act enacted on December 22, 2017 was mostly effective starting in 2018. That was not enough time for anyone to get a good understanding of all of its over 100 changes and the effect and relevance. The IRS has issued a lot of guidance, but there wasn't enough time to even get all of this finalized by the time any estimated tax payments for 2018 returns were due. If any practitioners have ever used Reg. 1.163-8T, 1.163-10T or temporary regulations under section 469, that's a reminder that guidance for some areas changed or added by the Tax Reform Act of 1986 are not yet final! And there are areas of many Code sections without sufficient guidance, such as Section 1202 added in 1993, but now widely used due to its now 100% gain exclusion (rather than the original 50%) and its reference in new Section 199A on the qualified business income deduction.