It’s not as relevant today as it used to be, that’s for sure. Originally it was to limit transcoding of 4K content (which used to be a lot harder), and also to avoid the HDR tone mapping issues with 4K content during transcoding, both of which are largely resolved with newer hardware and Plex software updates.
The only reason I keep them separated now is because most of the folks I share with can’t direct stream 4K content anyway, and so I only share out the 1080p libraries in Plex. It keeps bandwidth usage down and limits having to go to hardware transcoding, which can reduce quality and introduces startup delays. The library I use locally indexes both the 1080p and 4K content, so Plex will always prefer the 4K if it’s there.
If diskspace ever became an issue, I’d probably consider merging the libraries again.