Requires Lunar Client
This resource pack resizes the default Lunar Client background from 256x256 to 1024x1024, smoothing out jagged edges.
Supports every Lunar Client version
The change is more noticeable in 1.17 than in 1.8, presumably because the rendering was changed in between.
I have not tested any other version.
This project is not endorsed by Lunar Client and I will take it down if asked.