Is this sentence a passive voice with auxiliary omitted? Yes, it is. Normally there'd be an 'is' or 'was' there. In newspaper headlines or article/journal titles, sentences are often abbreviated to be as brief as possible, yet still get the meaning across.

Yes it is. The full sentence would be The ECB is expected to cut the deposit rate to at least minus 0.5%. Also, if you have any other questions feel free to check out my teacher profile.