In version 4.0.0 I at pipeline job level I could have used secret text credential that was used for authentication to read Projects and Repositories.

Selected it in the input Credentials (for build auth)

After update to 6.0.0 everything kept working as before (jobs still executing, authenticating etc) but dropdown Credentials (for build auth) became empty. 

Looks like only username/password is possible there now.

To make it work I had to enable basic authentication on the Data Center, make username/password credential with my username and access-token. 

Now it's ok.

But i don't want to enable Basic Auth. Server still perfectly works without it.

Can we get back Bitbucket PAT or Secret Text to the dropdown?
I understand this is kinda compat issue between two plugins probably. As list of creds displayed by Branch Source Plugin but issue appears after Bitbucket Integration plugin update

Yes, I also use SSH Credentials for clone