Adam, Virginia and I met. At the end of our meeting we looked into the fact that Proton is just one of many things we'd want to decommission.
At this point, it's worth for them to explore if they can come up with an entire list of things we want to shut down, with deadlines. Once they have that list, rather than focusing on one item at the time, I will propose an overall plan.

@Elitre
We have one caller with a generic user-agent making the majority of all calls. In order to learn more about who they are, since they do not follow our user-agent policy and we can not reach them, we will block their calls with a helpful and friendly message.

Example:
400 Error
missing informative User-Agent string with contact information as documented in User-Agent policy.

Any concerns about this approach?

Nice to have:
Including a way to contact us directly.
Do we have a generic email address or other method for contacting that we can include?

I'll answer in our planning doc, thanks.

In T329392#8694515, @VirginiaPoundstone wrote:

We have one caller with a generic user-agent making the majority of all calls. In order to learn more about who they are, since they do not follow our user-agent policy and we can not reach them, we will block their calls with a helpful and friendly message.

@VirginiaPoundstone: Is that T332061, or do you refer to another case?

It should be that one.
FWIW we keep working on this, and the next step is discussing with the PO, who is Olga.