I don't think the journey can be configured to consider that. In that case, you'd probably need to include a clause in your segmentation criteria to find and exclude contacts that have a certain value in that custom field. Or you might create a segment that finds all contacts with the opt-out option set and use that as a suppression segment for the journey, or add the opt-out segment to the start of all journeys, and set Containment method = Exclusion for the opt-out tile (and add another segment to collect contacts that should actually join the journey).
One problem with all of those methods is that once a contact starts the journey, that contact will stay on the journey until it completes, even if they join the opt-out segment while the journey is running. The do-not-email flags will stop sending emails to those contacts instantly.
