The Tax Cuts and Jobs Act of 2017 did away with recharacterizations of Roth conversions, but recharacterizations of new IRA contributions for the year are still permitted.
See Sec. 13611 of PUBLIC LAW 115–97:
https://www.govinfo.gov/link/plaw/115/public/97?link-type=pdf
and
§ 408A(d)(6)(B)(iii)
https://www.law.cornell.edu/uscode/text/26/408A#d_6
