Should I abandon an open math problem, if it hasn't been actively studied since the mid-1980s?
Based on your brief description, I'd say a more likely explanation for the lack of work on this problem is that people are stuck. Not that there's no interest in it.

Still, I wouldn't recommend this problem for a masters' thesis (or a PhD thesis). At least at the start, you should work on something manageable. If you hit a manageable problem or two out of the park, then you can start trying things professional researchers have attempted and failed.