The IRS website says you still can.

"You can file Form 1040-X, Amended U.S. Individual Income Tax Return electronically with tax filing software to amend tax year 2019 or later Forms 1040 and 1040-SR"

https://www.irs.gov/faqs/electronic-filing-e-file/amended-returns/amended-returns

"Any amended Form 1040 and 1040-SR returns older than three years, or Form 1040-NR and 1040-SS/PR returns older than the current year cannot be amended electronically. Amended returns for any other tax years or tax forms must be filed by paper."

https://www.irs.gov/filing/amended-return-frequently-asked-questions 

E-filed amended returns are more like the Model T in The Absent-Minded Professor, than the DeLorean in Back to the Future.  It's like the online Form 1024 application:  What you send is not 1's and 0's but something that shows up as a paper document for several reviewers to process.  I'm not sure it's a pdf, but close.  That's why,

"The current processing time is more than 20 weeks for both paper and electronically filed amended returns."