Should I report I might have been snubbed from a paper even if I don't have means to substantiate it?
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.