Why is this a "security" issue? People know that other people can see their "last seen" date, but usually they don't care so they left it. Hell, I leave it public as well. I couldn't care less about this tiny details.
Now, if you tell me that by leaving public my "last seen" date, people can get more data about me (e.g., with whom I talk to), well then yes, I would call that a security issue.
> if [...] people can get more data about me (e.g., with whom I talk to) [...]
It's quite possible, at least among mutual friends. If you have 2 people you know, who are also friends with each other, who have high correlation of online times, then, it's a signal that they could be talking with each other.
It's also amusing (insert a better verb...) that Facebook probably knows who's dating who based on frequency of messages on Messenger/WhatsApp/Instagram, location data ("It's the 3rd Friday night that their phones are close to each other in some venue"), and if they're on the same WiFi (A few hours later: "Well, it's 1AM, and after an hour of inactivity, James' phone is now connected to Jenny's WiFi, they did have their third date earlier...")
The most interesting part to me wasn't the technical details but rather the MAU, WAU, DAU stats at the end from random sample. Way fewer people than I would have expected use WhatsApp on a weekly basis.
This is in France, where text messages were free (unlimited with your subscription) long ago before we had internet on our smartphone.
Thus people use text message a lot and did use WhatsApp only to contact people outside from France
I remember that the facebook messenger was leaking the same kind of data and facebook said it's not a bug and didn't change it.