Do you mean it didn't feel mature before June 2016 or August 2019? Cause... those things are there now, so how can they be examples of it not feeling mature now? What's it missing for you now to feel mature?
(And also, lots of people and code definitely did left joins and multiple databases in Rails before those dates, without needing to add any sort of plugin/additional dependency. I did myself, plenty of times! Rails just added features to make them smoother and more complete. Which is actually examples of how mature it is, that it went back and made smoother things people were already doing in less smooth ways, isn't that what maturity looks like?)
Well, business don't jump on the latest version right away for many reasons (stability, bugs, upgrade woes...) Maturity is closely correlated to how long things have been around. Just because multiple database support was officially added on August 2019 it doesn't make it mature after that date. It takes a long time to achieve maturity - people to adopt and improve on that functionality without breaking things. We have been using multiple databases for years as well, done our own way, which will now have to be re-done in the upgrade to Rails 6. It's obviously not a plug-and-play replacement.
It's never clear how long gems will be maintaned for, and when a new gem (like multiverse, which is the multi-database gem I suppose others are referring to) comes out, it doesn't mean that businesses are going to jump on it right away. It needs to be around for some time so that support and its reputation start to build up a case to use it in production.
So I have been accessing multiple databases for years, without any third-party gems. But it's true I've been doing it in limited ways, only a few tables that aren't the 'main' database, the secondary database being managed externally.
The things that were difficult before (that I didn't do), that are now supported/smoother, include: migrations (schema-history-as-code) for multiple databases; and the really new exciting feature, different database connections used for read and for write. (i'm sure there are other platforms that support that, but I know it's not universal).
To me these seem like fairly advanced features, I am not sure how many/which other "competing" frameworks support these features; it's a matter of opinion whether these features are required for 'maturity' or not, I'm not going to argue about that.
But to be clear that basic ability to connect to multiple databases has existed since long before August 2019.
For the other feature mentioned as being introduced four years ago in 2016, trying to make the argument that it indicates a lack of maturity because it should have existed for even more than 4 years in order to call it 'mature'? I think it's clear there is just some axe-grinding going on there.
In general, whatever Rails faults are (and of course it has plenty) I don't think a lack of "maturity" is one of them. There are few frameworks/platforms more mature (and I would actually be interested in what examples you are thinking of). If I wanted to talk about the places where Rails isn't as mature as would be liked, it's not features that don't exist or features that were introduced "only" four years ago -- it's in some new features that IMO were introduced with insufficient stability and polish, like webpack integration. That, or ActionCable, are much better examples if you want places where Rails isn't mature. The examples in this thread are... silly.
(And also, lots of people and code definitely did left joins and multiple databases in Rails before those dates, without needing to add any sort of plugin/additional dependency. I did myself, plenty of times! Rails just added features to make them smoother and more complete. Which is actually examples of how mature it is, that it went back and made smoother things people were already doing in less smooth ways, isn't that what maturity looks like?)