I'm having success in 4.27.2 after following the instructions you have linked. How are you trying to bring up the Oculus system keyboard, by clicking in a text field right? Were you able to get it working in any other version of the Unreal Engine?
I'm having the same issue. I'm using 4.27.2 and the vr template project. I have added a text field to the menu widget. Also did the recommended setting for oculus plugin in project settings. Still no keyboard shows up.
It so happens that if I Launch the project on the quest2, the keyboard does show up. It is not working in the VR preview play from the editor. The other thing to see is if it would work in the oculus branch of unreal engine from github.
To test, put an editable text in the widget of the VR template (you can either put another box of text or just remove one of the texts). Enable the options described in the link you posted. Build the apk and test it on Quest.