Thank you!
I have reported this bug as Bug #1923253 “Unable to turn on or off the orca screenreader on ...” : Bugs : arctica-greeter package : Ubuntu .
Hope we will get it fixed in 21.10 or maybe earlier.
What is good - the Arctica Greeter on upcoming 21.04 can be replaced back to Slick Greeter by using the method for 20.10 manually if Orca is essential.