> Who, for example, still uses ISDN in 2026?

Most TV and radio stations.

Do they use it via mainline Linux kernel's ISNDN drivers though (and not something proprietary)?

> Most TV and radio stations.

I doubt it. And as I said, telcos have ceased new sales of ISDN and will be shutting down copper networks within 3–5 years.

Therefore if there are still TV and radio stations still using it, they will be forced to stop using it by circumstance, i.e. they will find their ISDN will cease working after the telco shuts down the kit in the exchange.