I wrote a custom syntax highlighting
.vim file that doesn't work. Would posting it and explaining what it's supposed to do be on topic?
What if I was able to write an MCVE?
In either case, would the question be more on topic here, or on SO?
Yes, I believe that such questions should be on-topic, subject to a quality threshold. There needs to be a specific question of what text should and shouldn't be matched, a specific broken example, and enough code to troubleshoot the problem.
Furthermore, the relevant code needs to be embedded in the question itself, not just a link to a third-party site.