I a lot of medication is used "off label", especially in psychiatry. I haven't heard of scopolamine being used for depression. All drugs have side-effects, here is one list.

Once a drug is approved by the FDA, most often drug companies don't want the added expense of getting it approved in another area. However, that doesn't mean that research stops, it is just research done elsewhere, including doctors offices.

There are anti-psychotics that are unapproved for depression but commonly used. I have read that SSRI's are being used off-label for some types of pain instead of opioids.

It certainly is worth talking to your doctor about. It might be helpful in finding a cause of your depression. Your experiences are interesting. Drugs like tricyclic meds actually inhibit acetylcholine, which scopolimine does so maybe it is not surprising it helped you.