Make a policy for adding new sbws relay keys

Split from legacy/trac#28085 (moved):

I think we should also come up with some policy to add/remove key/values. Even if all the ones we have so far seem useful, we are not using it.

I don't understand. Do you mean:

There are some keys in the spec that are not in the code. Or There are some keys in the code that are not in the spec. Or There are some keys in the spec and the code that are not being used by anyone.

For instance, do we have tickets of cases where these key/values were needed/requested?.

When we find this information, we should update the spec so it tells us why we need each value.