I can find classic either in project settings or filtering for class activities when I am using the community version (studio) but when I switched to the enterprise version, I could not find the classic activities anymore. I am using Studio Pro 2020.4.3, Enterprise License, Windows Installer.
UiPath Studio started to support Modern Design Experience in version 2020.10.
So I suppose there is no option regarding Modern/Classic in 20.4.x.
And, as so-called Modern activities are contained in UiAutomation.Next activities package in 20.4 as the following, there might not be interoperability.