I often wear a T-shirt when teaching some beginning programming classes that says (sing it to 99 Bottles of Beer on the Wall):
99 little bugs in the code
99 little bugs
take one down and patch it around
123 little bugs in the code
I’m not sure what makes you think that. And their casual attitude towards " Non Backwards Compatible" changes makes me shutter think how they backport bug fixes. But a brief looking around doesn’t show any evidence that they actually do backport bug fixes at all. Maybe I missed it while looking around.
But it’s worth mentioning bug fixes are backported in OH. That’s why we have 4.0.4. That’s four new releases of OH 4.0 backporting a dozen or so bug fixes including fixes for core, add-ons, and the UIs. But for new features yes, of course you need to upgrade to a new version.
It wasn’t forgotten. It’s now implemented via a rules (and it was implement as such within months of the release of the 2.x MQTT binding, I know because I implemented it). See Marketplace MQTT Event Bus for OH 2.5 and a very thorough tutorial. See MQTT Event Bus Publication [3.2.0;3.4.9] for a rule template (i.e. just install and configure, no coding required) for OH 3 and MQTT Event Bus [4.0.0.0;4.9.9.9] for OH 4+ (as you point out). I only reiterate this to point out that since OH 3, installing and using this rule template is no different from installing and using the old MQTT event bus add-on, and in many ways it’s even easier and definitely more flexible than the MQTT 1.x MQTT Event bus add-on since it’s not all-or-nothing (i.e. you can choose which items are on the bus).