OS/2 my beloved.
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In the end I wound up with basically the same application software as on my Debian desktop, except running on Haiku instead of Linux. Haiku is noticeably snappier and more responsive than Linux+X+Qt+KDE, though.
Runs on modern Intel/AMD CPUs, but limited to 32-bit and low RAM limits. The OS/2 source is owned by IBM and IBM won't talk to you unless a number with 8 zeroes is involved.