Index of wsrep system variables - Percona XtraDB Cluster
If the value is empty, the first node in SYNCED state in the index becomes the donor and will not be able to serve requests during the state transfer.
Crash recovery - Percona XtraDB Cluster
However, when a new node joins the cluster, node C will be switched to the “Donor/Desynced” state as it has to provide the state transfer at least to the first joining node. It is still possible to read/write to it during that process, but it may be much slower,
the document within section1 said the only one remain node cannot provide any service during SST, but it’s conflict with section2, which said the only remaining node still can serve request, but it maybe very slow, it depends on.
in my opinion, section 2 is correct.