You can still use "Add to My Drive" as follows.
The "Add" feature has disappeared from the main stage (context menu in the right-click) and is now a completely hidden command known only to old users.
1) Open the "Shared with me" section.
2) Click a folder or files to be added to another location. (it will be highlighted)
3) Press down "Shift + Z"
4) Can choose a location in a dialog box.
"Shift + Z" is still showing in "Keyboard shortcuts" page when clicking a Gear icon on the top-right of the web interface.
Google has quietly switched the "Add to my Drive" function to another "Add shortcut to Drive".
That function was released firstly to Gsuite account August, 2019, and It will be applied sequentially to each personal account too, now.
However, It's unclear how long "Shift + Z" will be living, whether "Shift + Z" also will be disabled in September this year, and what Google will do with the problems after September.
See details here > https://gsuiteupdates.googleblog.com/2020/03/shortcuts-for-google-drive.html
This forum also announced it 26-March, but Google Employee just pointed out a help page and didn't write anything about removing "Add to my Drive".
Google is a company that simply changes it as they like and leaves all subsequent trouble verification to the user.