After changes in MDEV-26971 (adding new features for progress reporting) and MDEV-28423 (fix for broken IST after MDEV-26971), the scenario with rolling upgrade 10.3+ -> 10.9 became impossible, i.e. it became impossible to upgrade without temporarily stopping the Galera cluster and performing an upgrade on each node separately. This is due to the implementation of parsing of tags passed through magic files during SST and with a strong discrepancy in the code of SST scripts between versions 10.3+ and 10.9.