Slack isn’t infinite. At some point there are no more old messages or no more new messages depending on which way you’re scrolling.

The problem is infinite content.

Pagination works just as well with infinite content.

Good. Use pagination instead.

What good does it serve to have to constantly hit 'next page' in the middle of a real-time conversation? That's a bad UX.

Maybe re-read the thread? A real-time conversation is not infinite content, it’s chronological and it ends when there are no more new or old messages. You shouldn’t use pagination, and I don’t expect this law or any similar law to enforce pagination on real-time conversations.