This is what i found on google
In Minecraft snapshots, experimental features can be enabled by adding their respective experimental IDs to the
server.propertiesfile.For enabling the 1.21 experimental feature on your server, you would need to find the correct experimental ID for this feature. You can usually find this ID in the documentation or the release notes for the specific snapshot version. Make sure you are using the exact ID provided to ensure the feature is enabled properly.
Assuming that you have the correct experimental ID for the 1.21 feature, you would add it to the
server.propertiesfile next to the line that says "initial-enabled-packs=". Here's an example:initial-enabled-packs=ID1, ID2, ID3, 1.21_experimental_IDReplace
1.21_experimental_IDwith the actual experimental ID for the 1.21 feature. Make sure there are no spaces between the commas and IDs. After making the changes, save theserver.propertiesfile and restart your server for the changes to take effect.It's essential to double-check the documentation or relevant sources for the specific ID to ensure that you are using the correct format and information in the
server.propertiesfile.