I don't know if I could go back to not having transactional DDL, partial and function-defined indexes, non-blocking ("concurrent") indexing, and so on...

I'm not sure where I suggested "Hey, long time Postgres users, you should switch to MySQL!". The point is it's a solid modern database and a completely fine option in 2020.

MySQL8 supports functional indexes and concurrent indexing fyi. It still does not have transactional DDL nor partial indexes.

Good to know, thanks. And I didn't suggest that you suggested that. :-)

What do you build that requires those things?

I've used them (via Postgres) to build large database-backed web applications (like Hey), realtime bidding systems (100mm+ records, 250,000+ requests/sec), and so on over the last 10+ years. Recursive CTEs as well, and MySQL finally has those as of a year or two ago. The feature gap is closing, but it's been a very, very slow process.

Is there a MySQL Roadmap somewhere? I cant seems to google anything up. Compared to Postgres which seems have way more information.

And Postgres still hasn't caught up to MySQL in multi / clustering solution. Or may be they dont ever intend to focus on it and leave it to third party.

Not that I'm aware of no. While the end product is released open-source, the development cycle itself is very much closed-source and there are very valid concerns about MySQL being owned by Oracle. MariaDB is a fully open-source alternative though I'd argue MySQL8 has surpassed MariaDB in terms of quality and features.