Last update date an be an indicator, without this info is hard understand what can be old and abandoned.
The date can give an indicator, after then admin can check forum or ask help / info about that old extension.
I think will be a useful info and thinking also for example on WordPress, some security plugin do just this: alert you when an extension is old more then a year. The final decision on what to do is always left to the administrator.
While it's hard or impossible to develop a function that can say to the admin if the developer have poor security practices, it's more easy just give the info about how how much far was the last update.
This is only my point of view, looking also what happen in other platform.
A community can have many extensions, know how much they are old in update can be a very useful info, I think otherwise I think admin cannot have info about how much old extension are.
Extensions can be created and abandoned.
Maybe will be useful also add an alert if the extension has been removed from the store.
In the future maybe some security guide line should be implemented in the "extension store".