I have noticed that this seems to be the case with a lot of "-able" words. When working, I occasionally pause to think whether they are "real words" or not, but they tend to be generally understandable. And in technical manuals, for example, when trying to save space, adding "-able" to replace "can be XXXed" helps a lot!I will guess that this is a case of dictionaries not being able to keep up with the pace of change.
I think that omittable meaning being able to be ommitted is in the process of being formally accepted into the language.
Perhaps because it is a rather unusual word and most people would prefer to say "X may be omitted" or "X is optional" rather than "X is omissible".Omissible!
Why is it that I don't remember seeing it so often?