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.
Thickness and weight might have been a serious issue 10 years ago but batteries have reached such high energy densities that the additional volume/weight becomes negligible especially considering how thin most phones already are.
It’s not really the battery thickness that is the issue it’s the docking mechanism that has to be robust enough for real world usage.