"For 2021, the client's 1099-R Disaster distribution was reported on form 8915-E in the tax return, and I elected to spread the taxable amount reported over 3 years. So is 8915-F for 2021 the form the second portion of the taxable 2020 amount to overflow into?"

For 2020 Tax Year, the form 8915-E applies for 2020 Disaster Years. Simply keep those two "years" in mind to know which Form applies.

8915-F is new. It is a Forever Form. It applies to Disaster Years 2020 and forward, for Tax Year 2021 and forward.