TL;DR

I think the author's GitHub account was hidden, deleted, or suspended.

Reasoning

The page https://github.com/substack also leads to a 404.

I found a July 2022 snapshot of the minimist github repo on archive.org. It looks like there were 43 open issues and 28 open PRs at that point, and the latest commit was 4 months prior, in March 2022. While 4 months isn't a long time, the number of open items may indicate that the maintainer didn't have enough time to keep working on the project. This is pure speculation on my point - I haven't tried to make contact or anything like that.

While the NPM unpublish policy and the NPM acceptable use policy define scenarios where the package could be removed entirely from NPM, it doesn't even appear to be deprecated. The package page is still up on the website.

GitHub policy allows for suspending or hiding a user account, or the user could easily have chosen to delete it for whatever reason.

My recommendation

Both the repository's README and https://security.snyk.io/vuln/SNYK-JS-MINIMIST-2429795 indicate that 1.2.6 fixes that security issue, so I would force that in your lockfile as soon as you have the bandwidth. If this is a transitive dependency, it may also be worth warning the maintainers of whichever package directly depends on minimist.

You do have another option, since the code is licensed under an MIT license. You (and/or anyone else who is interested) can choose to fork the code and build a fully-compatible replacement version of the package. Of course, you can also choose to switch to another argument-parsing library.