is ipv6 supported on any of the Xfinity approved 3rd party modems?

I'm wondering if Xfinity supports ipv6 devices connected to an approved third party modems? ...

See the last item on the "Details" page for the devices listed on https://www.xfinity.com/support/devices. AFAIK all of them support IPv6. If Comcast's support is not universal across its markets, it is at the least very widespread.

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I am using a Motorola (Arris) SB8200 and it supports IPV6.  You should visit the support page for your model modem; there should be a description there of how to enable it if it is available.  I have been on my current contract with xfinity for nearly 6 years, and it is only fairly recently that both IPV6 and DOCSIS 3.1 were enabled on the node that I am assigned to.  Still, any recent vintage modem made withing the last 4 to 5 years should support IPV6. 

For the most part, modems don't care about IPv4/IPv6. It's the router that needs to support it. I used to have a Zoom 5341J, and IPv6 worked on it. Comcast does use IP to access the modem for checking and updates and configuration. That may be v4 or v6, depending on the model, but is not relevant to the end user.

I see that a lot of modems show as being ipv6 capable. I have XB7 that has ipv6 working when not in bridge mode. In bridge mode however, the modem doesn't reply to dhcp6 solicit requests. Xfinity's support says that they don't support bridge mode even though they describe what it is and how to enable it here https://www.xfinity.com/support/articles/wireless-gateway-enable-disable-bridge-mode

I tried connecting my own MB8611 which is simple modem without firewall, wifi or any routers. As expected - ipv6 doesn't work and the there is no response to dhcp6 solicit messages. So my question here is more like if there is some minimum level of guarantee that Xfinity provides about their service and when that minimum level is not met - they would confirm a problem on their side and will work on resolving it. Versus the alternative where the response is - this isn't supported.

I have the mb8611, I can turn IPv6 on thru my router settings….

also in the mb8611 event logs mine mentions ipv6 provisioning mode

The provisioning mode message is for the Comcast internal modem access/control. It doesn't necessarily affect what you see in normal usage.