https://www.irs.gov/retirement-plans/ira-faqs-recharacterization-of-ira-contributions

"Effective January 1, 2018, pursuant to the Tax Cuts and Jobs Act (Pub. L. No. 115-97), a conversion from a traditional IRA, SEP or SIMPLE to a Roth IRA cannot be recharacterized. The new law also prohibits recharacterizing amounts rolled over to a Roth IRA from other retirement plans, such as 401(k) or 403(b) plans."

Recharacterization is basically a "do over" which is not the same as Backdoor Roth.