Unless your freeware is somewhat popular or used by high-value targets, its usefulness for conducting supply chain attacks is probably limited. If the domain name enjoys good SEO, I would rather expect that a speculator registers the name upon expiry and turns it into a parked page with sponsored links. More something like that.
If you don't renew the domain it will eventually be deleted and available to register by anyone (unless the registrar decides to keep it for themselves. This sometimes happens with valuable names).
My advice would be a) renew for one more (and last) year and b) update your website to state that the software is no longer supported after a certain release, and that the website will ultimately be retired. And even check that archive.org picks up the change. The idea is to signal your intent well in advance, and leave a record online.
And c) try to send notice to at least a few shareware directories possible, so that your application is flagged as abandoned/discontinued.
And perhaps make it a habit of signing code and commits with a PGP key too. Sign your code if you haven't already (and sign the retirement notice as well), publish your public key prominently on your website and upload it to a few key servers like https://pgp.mit.edu/ or https://keys.openpgp.org/
The idea here is to "plant your tent", and if anybody does anything bad with your former domain name, at least they cannot sign their misdeeds under your name (PGP key).