My research supervisor left the university and no one told me

Am I right to be upset that no one informed me that my supervisor was leaving/has left?

While it sounds like your subject (counselling psychology) is much closer than mine (mathematics) to understanding human emotions, I would suggest that it may not be productive to think of emotions in terms of "right" and "wrong". I don't know what your personal relationship with your advisor was, or what the norms/expectations are in your field/department with respect to supervisor relationships. While in most instances, I would expect a supervisor to let their students know personally if they are leaving the institution, it sounds like this may have been a sudden leave and supervisory expectations for masters theses can be quite different than for PhD theses. (In math in the US, the former is quite rare.) To continue...

she had informed the head of the department with my name and thesis topic and asked them to check in with me. I was never informed about this change. My supervisor didn’t send me an email to inform me when this happened in June and I still haven’t been contacted by anyone from the department

With the pandemic and other disasters, I would guess that someone accidentally dropped the ball, so you could be upset, or be understanding, or some combination of both, but I don't think that any particular "choice" of these emotions is "right."

Anyway, after you learned this, did you contact the head of the department? If not, try this and/or the graduate student director if your department has one. Presumably your former supervisor's goal was that the head of the department would try to find you a new supervisor under which you can continue your current project. I don't know how it is in your field/department, but in mine if your thesis was essentially done, we would just have someone else formally complete the supervision even if we don't have an expert in the area.

Is your supervisor out of academia or does she simply switch the university?

Is there some possibility at your university, that someone external be a second supervisor of a thesis?

If not:

Maybe it will be fine for her if you find some "official" supervisor at your university, without the expertise, but she will be for you there to answer some questions you have? It can be that at the end the work will be published, so this would be also beneficial for her.

I would contact her if such a possibility exists, if yes, I would then start to look for some supervisor at your university who is fine with this.

This was something I dealt with more than once as steward and executive officer for my Graduate employees union.

First of all, your feelings on the matter are completely valid. I know it's a scary place to be, and I'm really surprised that Universities don't have better established protocols for dealing with this. Try not to panic, and don't worry if you don't have a representative. There wasn't much the union could do in these cases except act as an official witness during some meetings.

Secondly, in the cases I encountered, the graduate students were always able to have another faculty member step in and take over as primary advisor. I'm pretty sure they all were able to defend on the same thesis topic they were already working on, mostly on-time. Your thesis is about the work you have done, so it should all still apply, even if the new advisor isn't necessarily specializing in that field. Some academic departments have faculty committees for student welfare or sometimes a graduate student advisory committee. Those might be helpful resources, if they are available. You could also try reaching to any committee members (assuming you have one), department chair's, or any other faculty you've worked with. I find that most faculty are willing and able to help students in their department succeed.

I'll caveat some of this by saying that I only dealt with cases in the sciences where most students were on some form of assistantship funding (either teaching or research). I know that not all departments work the same.