This email is important for you if you are running an instance of Wikibase.
As part of deprecating wb_terms table for the whole Wikibase codebase, the new term store is now wired to the Wikibase default setup and per the default it's going to read from the new term store.
It means that next time you update your Wikibase (to master or 1.35 once it’s out), you should run update.php  so it creates the new tables and migrate your data to the new term store. This might take some time if you have a large installation. If you’re not running the script, you might see errors appear.
The default value until the next mediawiki major release (1.35) is going to stay "read new, write both", meaning you will have both old and new data side by side but it reads from the new one. By the next major release after 1.35, we are going to fully drop wb_terms table from Wikibase.
Please note that there’s an exception related to search: if you are not using ElasticSearch, the search still uses the old term store (wb_terms table). That’s why the default is to write to both term stores for now until the search works for the new term store properly.
If you have any questions or issues, feel free to write me.
: https://phabricator.wikimedia.org/T208425: https://github.com/wikimedia/mediawiki/blob/master/maintenance/update.php