If the filename contains "/", the illustrator will save the file with "save as" command removing all the characters before "/", including it.
Example:
Saving with "save as" the file "My first project/version 1.ai" will end in "version 1.ai" on harddisk.

Also, the name of the file in document's title bar is replacing the "/" with ":"
"My first project:version 1.ai"

And, again, on Save as dialog box, the "Save as:" textbox where the filename is already entered, it shows up like this
"My first project-version 1.ai"

macOS Big Sur, Illustrator 25.0.1