It costs the Internet Archive $2/GB to store content in perpetuity, maybe create an account, upload your code as an item, donate $5 to them, and call it a day. Digitally sign the uploaded objects so you can prove provenance in the future (if you so desire); you could also sign your git commits with GPG and bundle the git repo up as a zip for upload.
EDIT: @JZL003
The Internet Archive has their own storage system. I would assume it caps out because they're operating under Moore's Law assumption that cost of storage will continue to decrease into the future (and most of their other costs are fixed). Of course, don't abuse the privilege. There are real costs behind the upload requests, and donating is cheap and frictionless.
https://help.archive.org/help/archive-org-information/
> What are your fees?
> At this time we have no fees for uploading and preserving materials. We estimate that permanent storage costs us approximately $2.00US per gigabyte. While there are no fees we always appreciate donations to offset these costs.
> How long will you store files?
> As an archive our intention is to store and make materials in perpetuity.