Forum Discussion
Sharepoint list can't exit classic experience
Ryan Kohn : I presume, there might be some customization happened on Display Form (Allitems.aspx), if any customization happened in Display form then SharePoint will not allow user to view the list in Modern Experience.
But if you are pretty sure that there is no customization happened in your Display form, then go to List setting -> advance setting -> scroll all the way down and there you can see option called List Experience, there you can select New Experience. this will overwrite the Default Experience.
IIRC, when I originally hit this problem that setting didn't help me.
Eventually I tracked this down to a bug in SharePoint (confirmed by MSFT support).
There was a workaround in that there was a training "#" sign at the end of the URL. If that "#" was removed you could get out of the classic experience.
EDIT: Eventually, this bug was fixed.