I can see having the ability to save an Ignore list, especially when you discover your edit is not going to finish in one session
I have no problem with it being discarded at the end of a session.
I was going to say: Keep Until I open a new title... Then I realized that I sometimes have 2 Sigils going (to copy bits-n'-pieces from one to the other) the saved state (locations) of Sigil can get muddled.