You’re right, this has come up frequently (and the initial plan was to have this in the API - as I believe I said at some point in the past). The fact is that this has not bubbled up the priority list, as we have focused in other areas. Priorities and direction (and time and effort) dictate a lot of these choices.
We recently published our plans for the future of the API Platform https://blog.twitter.com/2017/building-the-future-of-the-twitter-api-platform and our roadmap https://trello.com/b/myf7rKwV/twitter-api-platform-roadmap
As you can see, we have a lot to work on, and we are looking to provide better access to Twitter data and customer service opportunities. At the moment pinned Tweets are not a part of the plans, but I’m always keen to provide better information to developers so this remains on the backlog.
I’m not aware of a way to determine this data otherwise, so while I totally understand what you’re trying to achieve, this is not something in an API object that you could use right now.
Thanks for the question, and for the idea!