Can it be assumed that every star in an amateur astrophotograph has been cataloged?
Variable stars might challenge this.  A distant Mira type variable or recurrent nova could have been at its minimum when the (for example) Gaia catalogue was being assembled, but appear in an amateur photographs.
These stars undergo very large variation in brightness.  Mira can increase in brightness by 8 magnitudes, so it is not inconceivable that an uncatalogued Mira type variable could be found in an amateur photograph. Yet have been omitted from other catalogues by bad luck.
Recurrent novae are also &quot;variable stars&quot;. They also undergo large changes in brightness. As they spend many years at minimum brightness and then suddenly flare, there is a greater chance that they could have been missed in the Gaia and other catalogues.
However, these are rather special cases.  In general, any star that an amateur could image with reasonable equipment will have been photographed, identified and catalogued by one of the sky surveys. Automated image processing makes this possible. Many of the things that amateurs did in the 1980s and before are now done by robots and computers.
Supernovae have often been found in other galaxies by amateurs, with no catalogued progenitor (though supernovae might not be considered a &quot;star&quot;)