The behavior you describe with cell phones is also how overall internet access works in many geographic regions.

There are areas where residential internet is fast and reliable, but if you’re building a service that sucks on bandwidth/energy/throughput constrained links, you’re building a service that’s not viable for tons of users.

You're right. In the last decade or so the capabilities of the average internet user have degraded significantly. Should we handicap all software to accommodate these people? Profit motive says yes and incorporated entities surely will do so. But it is not the right move for human persons.

The people with bad internet connections are human persons.

The idea that to make something that works for those people we have to make systems that are worse for people with fast internet is a false dichotomy. We can (and should) make services and systems that work well for everybody.

How exactly is it handicapped? Last I saw, modern software like Matrix does everything IRC does and 100x more.

That's exactly it. Mobile phone users have to use things like matrix with extremely heavy protocol overhead and someone else hosting their client software and maintaining connection (for the vast majority it's element.io in browser using their owned matrix.org homeserver).