wsrep_provider_options pc.bootstrap cannot be set any more in 11.2.1

Dynamically bootstrapping a node was possible at least until 10.11.
We tried this as well in our Galera Training with 11.2 and it did not work any more:

MariaDB [(none)]> set global wsrep_provider_options='pc.bootstrap=yes';
ERROR 1238 (HY000): Variable 'wsrep_provider_options' is a read only variable

This is the behaviour in 10.11:

MariaDB [(none)]> set global wsrep_provider_options='pc.bootstrap=yes';
Query OK, 0 rows affected (0.001 sec)