Impress does not allow you to create custom user variables for use in presentations. A use case would be changing a client name or similar throughout a presentation. The option is available in Writer i.e. Insert - Field - More Fields - Variables But it is not currently available in the Impress menu.
sounds reasonable, still missing in master.
*** This bug has been marked as a duplicate of bug 53548 ***
Please always add the version info, available via the About dialog. With the current version 25.2 I can insert the author field, which should be filled from tools > options > user data. So "changing a client name or similar throughout a presentation" is possible. I made this request a duplicate of bug 53548 anyway since I guess you expect more fields, actually access to all variables defined in file > properties. => duplicate