I dont think the letter as been customized for this special situation, whether its worth your time to go in and edit as a custom letter, youd need to decide.

I havent seen anything about how to make these later payments...would a 1040V for 2020 suffice? Will IRS send a bill? What about the payment due in 2022?

And that is the challenge.  How does the client make the next 2 payments?  Hoping someone else will have a more constructive reply.

I have not seen anything from the IRS about how this is going to work either.  In a perfect world we would have instructions from the IRS and we would pass that info along to our affected clients.  I'm sure at some point the IRS will release guidance on this but until then, just keep a list of affected clients and update them with more information once we have it.

Picking up this thread -- is there any form on your 2021 returns a taxpayer must have if they deferred a portion of their SE taxes from TY 2020 and was responsible for paying half in 2021 and half in 2022? 

How would they document their required payment in 2021?

No, thats between them and IRS now, its a 2020 debt, due dates are 12/31.  it doesnt go on the 2021 tax return.

You can google:

irs pay deferred se tax

Remember that this is Deferred Payment. Not Deferred Tax. All the taxable income was already reported.

All they need to do is Pay, directly to the IRS. That search will give you lots of IRS articles; send the links to your client.