The DWP did not want to give the £20 PW to all UC claimants they wanted to give it only for new claims that involved claims due to Covid, this meant a change in law and the DWP wanted it rolled out quickly, therefore there was no time to change the law.
I received an email from 38 degrees informing me of the upcoming case I believe it will be some time at the end of July 2021. If the case is won and not appealed all legacy benefits should receive a backdated payment, to be honest the DWP are so inept I expect they will mess this up in some way.
Some people were paid tens of thousands in backdated payments for the IB transfer to contribution based ESA fiasco, then told they had to reduce the money to below the savings threshold within one year, or it would be counted against them.