AutoSave switch greyed-out.

this is from:

https://support.office.com/en-us/article/What-is-AutoSave-6d6bd723-ebfd-4e40-b5f6-ae6e8088f7a5?ui=en-US&rs=en-US&ad=US

"AutoSave is enabled when working on a file that is saved to OneDrive, OneDrive for Business, or SharePoint Online. If the file is saved to another location, then AutoSave is disabled. This will happen if your file is on a SharePoint On Premises site, a file server, or saved to a local path like C:.

There are other reasons AutoSave could be disabled as well. For example, if your file is in an older format like .xls, .ppt, or .doc., it will be disabled. It will also be disabled if your presentation is in slide show mode, or if your workbook contains password protection. If AutoSave is disabled, you can always hover over the AutoSave switch and a tooltip will tell you why it's off."

"A tooltip will tell you why its off" ...works great :)

My Document is saved to Sharepoint online or OneDrive

J.B.  I get what you did there, you redirected what I specifically said was a problem with a feature that had previously been working just fine for over 25 years that now REQUIRES the use of cloud services from Microsoft... well played sir.. well played...  

Unfortunately you've only repeated what other non-supportive and unhelpful responders have done before you, i.e. not provided an original or useful response.  I'm not intending to disparage you and I respect your response, I and others like me won't find it helpful.

You must understand there are a very large majority of product users who do not want to always require connection to their "one-drive" which has a local and a cloud component with "auto-save" functional recovery.  Yes, one-drive has a local drive component and a cloud based component {insert quote from Microsoft "for protecting needed files"}.  However I may not like or want to use One-Drive and only want to use my OWN disk where I don't have the potential of all of my files being captured by big corporate who "never" does anything wrong or illicit with customer files.

This is tantamount to saying someone who wants to have a survival kit for their home in case of disaster must use the disaster services warehouse NOT AT THEIR HOUSE to keep a disaster survival kit.  How wrong is wrong?...

I'm on a Mac and it works the same way. Auto-recover doesn't have the same functionality.

So if you want Auto-save enabled, you have to start by opening the doc in the online editor, then select "Open in PowerPoint", for example. Voila! The Auto-save is enabled. Until you open in a different sequence next time. And then you are back to the grayed out option.

this is from:

https://support.office.com/en-us/article/What-is-AutoSave-6d6bd723-ebfd-4e40-b5f6-ae6e8088f7a5?ui=en-US&rs=en-US&ad=US

"AutoSave is enabled when working on a file that is saved to OneDrive, OneDrive for Business, or SharePoint Online. If the file is saved to another location, then AutoSave is disabled. This will happen if your file is on a SharePoint On Premises site, a file server, or saved to a local path like C:.

There are other reasons AutoSave could be disabled as well. For example, if your file is in an older format like .xls, .ppt, or .doc., it will be disabled. It will also be disabled if your presentation is in slide show mode, or if your workbook contains password protection. If AutoSave is disabled, you can always hover over the AutoSave switch and a tooltip will tell you why it's off."

"A tooltip will tell you why its off" ...works great :)

My Document is saved to Sharepoint online or OneDrive

This apparently is not a fault, but is a design feature of an update to Excel. I understand that it is proposed to restore normal auto-save for ‘locally stored’ files in a future update.

Why this was done is a mystery!