Saving page in web archive format
Old Opera versions had page saving and opening option in web archive format (by single file). It was very practical and easy-to-use, otherwise saving whole page in modern versions forms an additional folder with many small files and complicates operating after all.
What about restoring of this important function?
I guess you talking about mhtml file
there is a flag
enable it and opera will save pages in that format (single file)