Here are my test results, and there are some real anomalies here. Bruce, good question about whether only mods can see the profile. The answer is yes. However, note the other discrepancies and inconsistencies.
When looking at the message list (i.e., without search filtering), what you see is consistent, whether the past member banned or non-banned, and whether or a mod or a non-mod is viewing, namely: the email address and display name are always visible, both at the top of the message and in the attribution below the message in the list. The display name, however, is not clickable so the profile is not viewable.
However, in search results on the message list, the results are the same as for the message list except:
(1) if the member is banned, the display name and email address show up below the message; and
(2) if a mod is viewing the display name is actually clickable and takes you to the profile.
I agree generally with Bruce that none of this - neither email address nor display name (let alone, clickable display name taking you to the profile) - should be available for a past member. Weirder still, although more an anomaly than a danger, is that only if the member is banned do mods see a clickable display name leading to the profile.
I've checked and re-checked this until my brain is fried, and I think it's all accurate, but hopefully others will double-check to confirm. I haven't thought a whole lot about it but will try, even if further thoughts occur to me, not to post a bunch of p.s.'s for now.
Messages are the sole opinion of the author, especially the fishy ones.