Dammit, I got the same notification and updated Legacy thinking it was still Legacy but it's v10 and I'm pissed! The primary reason for me using Legacy are tabs. Like others have said, this makes my workflow so much easier as I typically have 10 Evernote tabs open to the same ones I most frequently use, so all I have to do is click a tab and start writing.
Now, my software is still called Evernote Legacy. Does anyone know why there's regular and Legacy if both have now been updated to v10? Makes no sense.
Other things I don't like in v10, the interface elements are big. I would love a feature to minimize the interface so that it's mostly note taking and what I prioritize such as font, size, and color. Oh, and don't get me started on this new font, it's clunky and the line breaks are a waste of space.
Anyway, I messed up so will try playing with this the last time I had to go on Legacy. At least my work computer is still on Legacy. The good news is, at least PDFs and attachments are back. And friends have been trying to get me to convert to Notion, this might be the right moment.