Hi
Welcome to Microsoft community.
Your question should be able to change it in Roblox settings.
Check it in other games.
It's conceivable that some players have recently started playing a Roblox game that doesn't allow them to use shift mode. Check if it's still compatible with other games. If it works with other games then it's likely that the chosen game may not support it. If this option does not work with any game, then it's probably an issue with the settings, and players need to explore further.
You can try changing the camera mode to see if it works.
Keep us posted if you encounter any challenges along the process.
Best regards
Derrick Qian | Microsoft Community Support Specialist