T-Mobile declared that the LG V10 would get Marshmallow starting this week. Apparently the process has begun.
T-Mobile hasn't updated the support page yet, but the changelog is sure to appear before the goods reach everyone.
T-Mobile has since updated the support page with the latest changelog. In addition to Marshmallow, the V10 is also getting Advanced Messaging and T-Mobile Video Calling.
The OTA is going out in waves and can take weeks. You might get lucky when you check manually, or you may have better luck installing LG Bridge instead.
How much you're willing to wait could depend on how eager you are to extend battery life or adopt a microSD as internal storage. When it comes to Marshmallow, under-the-hood changes are where it's at.