The bug
When a certain world from an older version is upgraded and then entered, the "Experimental Settings" warning is shown even if there are no experimental world generation settings involved. This is also the case in previous pre-releases.
Debug report
A consistent method of reproducing the bug is still yet to be discovered, but if it contributes to the bug report, a debug report of an upgraded world can be seen below.
Please mark the bug as Invalid. The "Experimental Settings" screen was actually caused by a data pack that attempted to add a customized dimension.