Given T251378, I'd propose to move one memcached shard to Buster. The goal is to verify and test with production traffic that the configuration for Buster works as expected. There has been a lot of testing in the past, but I am pretty sure that some tuning will be needed for mc10xx. The idea is to work on one shard for the moment, and then upgrade all the others when we'll be in a good state.
Note the chosen mc10xx host should be removed from the Redis MediaWiki pool. It should be sufficient to check the MediaWiki config and the nutracker's one. Since a nutcracker restart is required, this might cause some user impact.