Operating System upgrades are, typically, the places where implementations of new Internet protocol standards are introduced, TPITTS437.
Sure. There was a change in either the default way Apple Mail formats authored emails, or in the implementation of a newer version of the Simple Mail Transfer Protocol (SMTP) Internet standard used to transfer email from Apple Mail to your chosen SMTP email server (going to a Microsoft Exchange email server seems to be the only troublesome link, here), or both; that seems to have triggered the issue that already existed in Microsoft Outlook.
As for attachments, to emails, they are still attachments! That hasn’t changed.
The only problem is that Microsoft Outlook prevents you, the user, from performing bulk attachment operations (such as bulk-saves) based solely upon the way Outlook chooses to display (render) your received email.
So far, none of us have seen any other email client that misbehaves, in this way.
This, and the various workarounds, simply emphasize the fragile nature of any means of trying to get Microsoft Outlook to “do the right thing”™️.
Such will continue to be fragile (subject to being easily broken), until Microsoft fixes this misbehavior in their Outlook software.
Unfortunately, Microsoft is highly unlikely to fix this issue, unless y’all let them know that this misbehavior causes you difficulty.
Y’all are also encouraged to provide Feedback to Apple, but anything they may do will remain fragile, until Microsoft Outlook has this issue corrected.