"A number of temporary tax provisions expired at the end of 2016. Although some temporary or expired provisions were addressed in the 2017 tax revision (P.L. 115-97), most of the provisions that expired in 2016 have not been extended beyond their 2016 expiration date. Some have suggested that a tax extenders bill enacted early in 2018 could retroactively extend expired tax provisions. Alternatively, another policy option is to allow expired tax provisions to remain expired."
CRS Insight, IN10848
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html