Are archive.ph links essentially plagiarizing subscription content?

Yes, with the caveat that the articles are generally “you can read up to N articles per month for free (or more if we are unable to track you as a repeating visitor)” or similar. (IANAL)

It would only be plagiarizing if you took the content and presented it somewhere as your own without attribution.

What they are doing is bypassing a paywall, which should clearly be the point of the links. You can decide if you have an ethical problem with that.