Take this question for instance. In my opinion is not about the web interface functionality, but about how versioning software works, so it should be closed.
Should these questions be left open? Or should they be closed?
If the tasks are done by using the web front then is on-topic here, if they are done by using installable software they are off-topic, but...
It's very likely that knowledgeable people (meaning, programmers that use this tools every day) able to answer this will be participating regularly on SO. On February 23, 2019 https://stackoverflow.com/tags/bitbucket/topusers reports 5,627 questions while https://webapps.stackexchange.com/tags/bitbucket/topusers reports 36 questions.
If the question doesn't mention what front-end is being used, then it should be closed as too broad, I think.