https://sigil-ebook.com/sigil/tips/#...is-really-gone
Why it was removed ... WYSIWYG editing is live HTML (not XHTML) editing internal to the browser engine (Chrome in this case that is embedded in QtWebEngine) and not under our control. It can and often does generate code that appears to work but is NOT valid xhtml required by epub and will not pass flightcrew or epubcheck that most publishers require.
The illegal code that can be generated by the old BookView was a constant source of pain that consumed much energy and took away from Sigil development itself. Since it is internal to the QWebEngine browser engine we can not fix or change how it operates.
Therefore we removed BookView and replaced it with a separate tool (PageEdit) for those who still want some of that functionality that may be still useful for final proofing and very minor edits.
Using CodeView and Preview mode will allow epub developers to see the impact of their code changes immediately without the need to toggle back and forth. That way correct xhtml can be generated and epubcheck detected errors can be easily tracked down and fixed. These were determined to be more important for the majority of professional epub developers.