In rMW36f4daf32c591d6b7e2435629fc6e431398b641a, MediaWiki's DBAL was adjusted to reject any write query on read-only DB replica connections. This poses a problem for extensions that use temporary tables in their queries, as such queries now have to be executed on the source DB rather than a replica to work around this fact. An example of such an extension is Semantic MediaWiki, whose QueryEngine uses temporary tables extensively in serving reads. The current situation, where all writes, including non-persistent ones, must be executed on a source DB connection, causes scalability issues since it's no longer possible to distribute these queries between multiple replicas.
In Fandom's SMW setup, we have configured the extension to use replica DBs (MySQL 5.7 running with --read-only flag set) for its reads, including the temporary table operations, and we have not observed any issues from this configuration. Thus, I think it may be worth to again permit temporary table operations to be executed on DB replica connections as well, to allow for better scaling of deployments that utilize extensions that need them.