I guess it depends, what would your ideal outcome be (assuming getting the work you did before back isn't possible)? Do you want a redundancy payout, a different job in the company, a different job not in this company or to coast quietly doing not very much as long as possible (this is obviously risky but surprisingly possible for a long time in some companies).
If what you really want is redundancy money I think I would be documenting clearly the aspects of your role that no longer are required and what % of your job description no longer exists and why (e.g. task A no longer required, system B replaced by system managed by another team, task C now done by other team), then I'd have an honest conversation with your director using that evidence, making it clear you aren't annoyed/about to stick in a huge grievance about being 'left off' the previous round of redundancy conversations (even if you are a bit annoyed in reality) because this will probably be a distraction, and say something like 'look, it really looks here as though when the merger happened my role should have been included in the redundancy consultation/discussions at that time, but we didn't know how things would play out, can we speak to HR about how we correct this process, I'd be happy to explore voluntary redundancy options if that makes the process easier' or something like that. You have to play this one carefully because some managers/companies will happily continue to employ someone and pay them their salary in exchange for very little work, rather than going through what they perceive as massive hassle and expense to do redundancy, I think that's mad personally particularly if the person actively wants to go but that's how some people think, so you delicately need to suggest redundancy is the easiest option for him, if that's what you want of course.
If what you want is to stay in the company then it might be a bit different, if you have one of those broad generic job descriptions I'd use that as an opportunity to try and broaden your scope, get involved in whatever other projects you can, get yourself out there a bit, maybe see if anyone in the wider team or organisation is up for coffee chats and give you a bit of mentoring, they may have roles open or can give you some work to do. Apply for other jobs if they are advertised internally too, in a weird way it might actually work in your favour to not have been branded with the 'at risk of redundancy' label, in theory if you are going to be made redundant you should be treated with priority for any suitable alternative roles in the company but a lot of managers really try and avoid taking redeployment candidates if they can, somehow thinking they're not good performers or something, most unfair but how it is sometimes.
Or you can just give this one up as a bad job and brush off your CV and get going looking for other jobs ASAP, use your free time to study up on whatever might make your CV more attractive (online courses or whatever) - you could do this in conjunction with either of the above options?