Should I report I might have been snubbed from a paper even if I don't have means to substantiate it? For my thesis, I did some programming for a project. My supervisor seemed to be very happy with the results when I was there and promised I was going to be in the publication. Problems started when I graduated and moved abroad. My supervisor said they couldn't replicate my results and started pushing me to continue into the project. I left all my scripts properly organized with README files and was available to answer emails and connect remotely to do minor fixes, but I was already fully dedicated to my masters and didn't have time to do extensive extra programming. Indeed, I was doing field work and didn't have access to computers for most of the time. After almost a month in which I was out of reach, I checked my email and found a communication claiming I have stalled the whole project and lead to its cancellation. I let things pass for almost two years, not having much contact with my then coworkers or supervisor, and just yesterday found the paper was indeed submitted around six months after that final communication and taken into publication! And I'm in the acknowledgements albeit for a menial task that anyone could do in a couple of hours! 

I can't be sure if there was misappropriation of my work since the code of the project hasn't been published. They might have rewritten the code, but they might have also indeed used MY code or parts of it. Unfortunately, I don't have a copy of my work, since I was doing everything remotely in the server and I now fear I wouldn't be able to substantiate my allegation to the editors in case they ask for prove to my claims (I know, I should have kept a copy for myself, but I never expected this to happen at all...).

I noticed some red flags before leaving but didn't think they were of real concern. My supervisor seemed to be resented at me leaving instead of staying in his lab. Although my supervisor is very good in his field, I didn't like the overall way in which he treated us, his students -- he is the kind of supervisor that would yell and call you stupid in front of your colleagues –. This is one of the reasons why I decided to leave the lab and even change field. I don't know if this affected his intention to include me as an author, but in any case, I think they should have informed me I was going to be mentioned in the paper, and they didn't. 

Is this among the normal things that happened in academia? Should I tell the editors either they include me as author or they drop any mention of me in the acknowledgements? Also, among the authors are other students for whom I have real appreciation and I don't want to spoil their names in their first publication. Should I let things just pass and be forgotten? (The paper has been published already for more than a year and haven't been cited once. -In my opinion it's a good paper, but not a breakthrough). What would be the normal thing to do in academia?
Unless you outright had years of valuable work stolen out from under you that imperils your future career in your field, and you have strong proof of what occurred that would establish to an outsider that clearly something very seriously wrong was committed (way, way past a snub), I'd "count your blessings". From the sounds of it, you have made a clean break from the field, the lab, and most (or all) of the people. There's no huge ethical issues with the paper itself (no fraud, abuse, etc), so you don't have to worry about guilt by association.

You don't need letters from them, you don't need to collaborate in the future, you don't need to even see them passing down the hall. It really sounds like an amazingly good outcome, with the worst-case scenario being you didn't get co-authorship on a paper no one particularly cares that much about. And demanding to be removed from the acknowledgements would make you look like a bit of weirdo, from an outsiders perspective - "give me authorship or don't acknowledge me at all", honestly, will not give anyone a good impression of you to anyone. No upside to be had.

As of right now, from an outsiders perspective, it sounds like did reasonable work as an undergrad, and moved on. You were a bit unresponsive (not checking email for a month tends to be uncommon), but quite understandable given the big changes - though it would not have been cool to be aware of pending collaboration and not even tell your collaborators you are not going to be "online" or reachable for weeks on end. People in the lab may have been disappointed that you weren't more interested in continuing the project to publication, so it sounds like there was some miscommunication, misaligned expectations, and you weren't all on the same page as to where you stood on the project now that you graduated and moved on to something not strictly related. I don't know what conversations you had together, so we can't determine if someone was just being demanding or if you had indicated interest in continual involvement but then effectively didn't follow through. Not our business, but something you might consider for yourself.

Given the lack of major downside, it would be a great opportunity to learn how you might avoid a situation like this in the future, because you'll of course eventually move on from your current position and most people don't perfectly have all work tidied up and permanently finished when they leave. Reflect on how to set clear boundaries, avoid making promises (expressed or implied) you aren't dedicated to following through on, set clear limits to what you are agreeing to, communicate your availability to collaborators, have direct conversations with people to ensure everyone has matching expectations, etc. These skills will be invaluable to your future career, so no time like the present to improve on them, learn from past experience, and build up to a more happy and productive future.