It had been in the works for a while, but now it has formally been adopted. From the article:
The regulation provides that by 2027 portable batteries incorporated into appliances should be removable and replaceable by the end-user, leaving sufficient time for operators to adapt the design of their products to this requirement.
There are open source operating systems that are Android based which will update to the latest versions and fix bugs and suchwell beyond the manufacturer’s support, lineageOS for example.
A tricky part is that some apps detect an unlocked bootloader and brick themselves, which effectively makes it impossible to use those apps on such devices. And while I don’t think rooting is a strict requirement for installing LineageOS based off a quick search, rooting also has this problem (and at least last time I installed a custom OS many years ago, I recall either having to root or thinking I had to root).
Your information is a few years outdated. lineageOS neither comes rooted, nor does it offer a native way to root anymore. Magisk became a thing with a whole community around it. It’s an unlocked bootloader hider, root manager (and hider), and a system patcher, all wrapped up in one tool.
With Magisk, you give root access to the apps that need it, hide root ability from apps that require non-root devices (those apps do that by pretending to need root). Also, the Magisk app can rename itself, which is important as some apps check against the name itself.
The future challenge is with Google trying to force hardware identification (Apple style). I have not been following developments regarding that though, since as others mentioned, my X years old phone is still serving me perfectly, and I have no intention to upgrade any time soon.
Assuming someone makes a ROM for your device. This is often the case if you go with one of the most popular models but less popular devices might simply see no development effort – if the developers can even get their hands on the drivers and other necessary parts to build Android for that device.
It’d be great if manufacturers had to release all of the stuff necessary to run AOSP on their devices but I doubt it’s going to happen.