Yes, the same applies to newspapers and magazines online. We need a law demanding a legal digital deposit at least at the national level.

It is somewhat trivial to devise an API to be integrated in the publications pipelines to automatically and transparently submit new and modified articles to a central repository.

We are not (yet?) living in a post-national world, but the simple digital solution is to require a digital copy to be sent to the national library, as some nations have done; since national libraries in any case are now all heavily working on digitizing their pre-digital assets and making them available online.

In the US, the mandatory deposit requirement likely worked pretty well with traditional book/magazine publishers and music labels. Outside of that, I expect a huge amount slipped through the cracks. There's no real enforcement AFAIK and I imagine most who are independently publishing or otherwise working outside of conventional channels don't deposit.

Internationally, mostly https://www.lockss.org/ and https://clockss.org/ .

In Norway you are required to allow the national library to archive your websites, but I assume there's a lot that's slipping through the cracks ...