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Hello Ben,
Thank you for posting in Microsoft Community. We are happy to help you.
I understand that you are having trouble finding the form customization options in Microsoft Lists.
It is correct that Microsoft has made some updates to the form layout in Microsoft Lists. The new form layout is now in a central modal instead of a sidebar.
Regarding the Manage Forms feature, according to the information in Microsoft 365 roadmap, it is currently in development and not yet available.
On one hand, I recommend you keep a close eye on the Microsoft 365 roadmap page to get the latest information about the feature.
On the other hand, when I test in Microsoft Lists, I don't find the previous way to edit the layout/styling of the form. I'm afraid it is not available to use now. We apologize for the inconvenience caused.
If you are willing, you may try customizing the form layout and styling by using Power Apps. You may create a custom form using Power Apps and then embed it in your Microsoft List.
If you want to know more about Power Apps, I sincerely recommend you try posting a new thread in the dedicated channel Microsoft Power Apps Community, members and engineers over there are proficient in the knowledge of Power Apps customization queries. They will focus on the specific situation on your side and provide specific suggestions for you.
Let me know if you have any further questions or concerns.
Thanks for your understanding and cooperation! Hope you have a nice day! Stay safe and healthy always.
Sincerely,
Tina | Microsoft Community Moderator