Mainframes are still holding on in use cases where a single server having continuous uptime is vital. They're designed to have uptime measured in decades, so even components like the processors and main memory have hot spares available and can be hot-swapped without interrupting the OS or running services. They also have continually running system monitoring and diagnostics at the hardware level (not running as an OS service) that will alert both the owner and IBM if they detect some sort of hardware fault. IBM has supported Linux as a first-class OS option for their mainframes since the early 2000s.

From a developer perspective, s390x is also the last active big-endian architecture (I guess there's SPARC as well, but that's on life support and Oracle doesn't care about anyone running anything but Solaris on it), so it's useful for picking up endianness bugs.

Another interesting thing is that the only two 32-bit architectures left supported are armel and armhf. Debian has already announced that this will be the last release that supports armel (https://www.debian.org/releases/trixie/release-notes/issues....), so I guess it'll be a matter of time before they drop 32-bit support altogether.