Yes they are. The only catch is - getting them to work in control-plane is more difficult, but since your flexvolume plugin worked in 3.11 where controller-manager is already conainerized, it may not be so for your particular use case.
[DC]: if you don't mind, curious to understand why you think in v4 is harder to get it working with the control-plane?
The flexvolume is for cifs and in order to work needs to:
1. Have the cifs packages installed
2. Have the user's kerberos keytab available (we're not allowed to use usernames and passwords)
on 3.11 we're managing this with a combination of FreeIPA (every node is a member of the ipa domain), Ansible and OpenUnison. Given 4.x's reliance on a container os (RHCOS or FCOS) my assumption was this wouldn't work anymore. Is that assumption wrong?