There is currently no way for a user to know when a page corresponding to a summit log they have signed has been deleted.
This doesn't need to be fancy - I don't need an email alert or pop-up notification. Perhaps there is a way to just show the deleted entry "grayed-out" in my "Summit Logs I've Signed" section for instance.
Use Case: Sarah signed a summit log in 2009 for a mountain she really enjoyed. For some reason, in 2013, the page owner just decided to get up and delete the page. Currently, Sarah has no way of knowing this (the deletion) has happened. However, if Sarah had a way of having the deletion brought to her attention, she is likely to dig up her photos and maps, do some research, and rebuild the mountain page, complete with appropriate attachments to area/range and routes.