> I would find even a statement from Signal like "we can't access our users content" to be tenuous and overly-optimistic.

I don't really agree with this statement. Signal literally can't read user data right now. The statement is true, why can't they use it?

If they can't use it, nobody can. there are no services that can't publish an update reversing any security measure available. Also doing that would be illegal, because it would render the statement "we can't access our users content" false.

In Slack case, it is totally different. Data is accessible by Slack systems, the statement "we can't access our users content" is already false. Probably what they mean is something along the lines of: "The data can't be accessed by our systems, but we have measures in place that block the access to most of our employees"