The document is incorrect

Index of wsrep system variables - Percona XtraDB Cluster

xtrabackup-v2 : Uses Percona XtraBackup to perform SST. This method requires wsrep_sst_auth

this section is incorrect, because wsrep_sst_auth was deprecated and removed from 8.xx version, and click the link within document, it also doesn’t redirect to a good location.

You are correct. I’ll open a ticket with out documentation team to get that section removed.