For comparison, Page watch feature retains both pages (since old page may not necessarily become a redirect, although that's the common case). I think for Topic subscription it's sufficient to just move subscribers to the new page or it should do both for parity with the Page watch and user expectation.

How does one subscribe to a page? I thought subscriptions were only for level 2 headings.

The way heading subscriptions work, is the subscription is actually to the first comment's signature's username and timestamp, not the heading name itself. So hypothetically you can copy paste the first heading and comment to a new page, and you'll end up subscribed on that page too if you're subscribed on the first page. The idea is that you can change the heading name without losing your subscription.

In T373543#10101499, @Novem_Linguae wrote:

How does one subscribe to a page? I thought subscriptions were only for level 2 headings.

https://mediawiki.org/wiki/Help:DiscussionTools#Page_subscription

In T373543#10100978, @Ammarpad wrote:

I think for Topic subscription it's sufficient to just move subscribers to the new page or it should do both for parity with the Page watch and user expectation.

A common case for not keeping the redirect after the move is archiving: some people archive talk pages by moving the live talk page to an archive title from time to time and then turning the old title into a new, fresh talk page. In this case, it’s more important to keep track of the title than to keep track of the page itself (which is now an archive), so I think the best would be mirroring the watchlist behavior and subscribing the user to both pages.