To be honest I rarely use "Open page number" in XnView 1.9x but I still honestly found it very useful. It is a feature applicable mostly to Animated files (GIF, APNG etc) and Icon files (ICO, ICNS etc) but also some TIFF image files too. It is neither a big feature to implement nor an important feature, just something to consider reviving for XnViewMP
.an image mock-up:
