It looks like my old Instagram account has been deleted, whereas I still have a confirmation of my account creation (in 2012) in my e-mail inbox.
Yes, Instagram tends to delete accounts that have been inactive for a prolonged period of time.
I found this on their website:
"We encourage people to actively log in and use Instagram once they create an account. To keep your account active, be sure to log in and share photos, as well as like and comment on photos. Accounts may be permanently removed due to prolonged inactivity, so please use your account once you sign up!"
You can check out the page here