I've been asked many times in the past for a method to show the classic SharePoint "Tree View" for SharePoint Document Libraries in a Modern SharePoint site. Today I was asked again and opted to make a video about it because "Sharing is Caring" and we all need some caring right now.
You can read the companion blog post for this, here: regroove.ca/brainlitter/2020/03/15/how-to-get-a-tree-view-in-a-document-library-in-a-modern-sharepoint-site/
I open with why you might want to still avoid it and I even propose sync'ng the SharePoint Site with your OneDrive for Business Client and leave the SharePoint site alone ... but in the end, I give in for my dear old friend Tony and lay out the goods because I love him.
The short version is:
1. Set the library mode to “Classic Experience”
2. Go to the (now secret) NavOptions page (_layouts/navoptions.aspx) and enable the Tree View (caveat: how long this remains available, I have no idea…)
Enjoy. I hope this helps someone.