It happened to me. I got out of it by studying open source tech stacks and building side projects until I could get a job away from the proprietary stuff. It wasn't ideal, but I managed to pull it off.

I'm mostly playing devil's advocate with my replies. It's generally better to stick with mainstream -- and ideally open source -- tech. But I don't entirely agree that proprietary tech is only beneficial to the company in these scenarios. They are taking risks that the employee can take advantage of. Heck, for me, that proprietary company still contacts me on occasion to see if I'll come back.