ViewState Editor by Mike Smith (PortSwigger Web Security)
This was because (quoting PortSwigger Agent),
the tab has been removed as it’s not used as much as our other views [like Params and Headers].
After a lot of feedback from the community
, the functionality is now available as an extension. Using this extension allows seeing the decoded ViewState for requests in Proxy History / intercepted Proxy requests.
Don’t forget to use this extension while testing ASP.NET web apps 😉