The bug
Every time the game is updated, existing experimental worlds will once again show the “Load experimental world?” warning when joining them. The message stops appearing in subsequent joins, until the game is updated again.
How to reproduce
Download a world that uses experiments
Join it and notice the warning
Leave and join it again and notice no warning
Wait for the game to update
Join it again and notice the warning
Expected result
The warning should only appear when joining the world for the first time, or when additional experiments are enabled
Observed result
The warning appears every time the game is updated
This is due to every drop having an experimental toggle so 1.21.60, 1.21.80 and 1.21.100 have experimental toggles for the next game drop. As well as other experiments for addon creators and gameplay experiments like villager trade rebalance, BETA APIs, custom biomes and data driven structures etc..