I didn't get this e-mail either and this isn't what the SW website says. They should update that.
When I canceled my flight on April 2, 2020, my e-mail stated I had to use my voucher by January 3, 2021 which was 1 year from the date I booked my flight. Today I went to the SW website and it states that they have extended the Voucher period and my Voucher now expires June 30, 2021.
I had a United flight that I canceled as well and initially my Voucher also stated it had to be used by January 3, 2021 but now the United website states the Voucher is good for 24 months from the original date of issue.
As an aside, I should have waited out United because I would have gotten a refund. I canceled when my cruise was canceled. However I have since found out that my flight to Floriday on 4/27 was changed after I canceled it. Because of the change, which resulted in a long layover in Chicago instead of a direct flight, it wouldn't have worked for me which means I could have gotten a refund. I may contact them anyhow and see what they say.