Sure, you use a dev site to develop themes and plugins, but those are files. They have nothing to do with the content in the database.
What? Installing plugins and themes writes data to the database. I may not have actual site content locally, but stuff does get written to the database that I want to move with me when I deploy to production.
I've worked at many dev agencies and never heard of a single time someone installed Wordpress on the production server.
It's just suuuuper frustrating when an update to a major platform like WordPress is deployed and it's not backwards compatible.
At least make it an option to use utf8mb4, not some automagic thing that happens. If it's going to do something that makes my database not work with older MySQL versions, let me decide on if I want it to do that or not, because maybe in the future I will have to move the database to a server that has a MySQL version < 5.5.3. Who knows? At least until MySQL < 5.5.3 is extinct.