Nios II SBT for Eclipse is not included start from Quartus Prime Pro and Quartus Prime Standard edition v19.1 onward.
You may have to refer to the workaround in KDB as below:
Why is Nios II SBT for Eclipse being removed from Quartus?
Is Intel replacing it with some other Nios II Software Development Tools?
If so what are the new Software Development Tools?
There is an change in replace Cygwin to WSL.
For question "Is Intel replacing it with some other Nios II Software Development Tools?":
No, there is no other Nios Softwrae Development Tools replaced.
For more information, you may need to refer to the document as link below: