It looks like the last machines of each architecture were released:
Alpha in 2007
HP-PA in 2008
m68k in pre-2000 though derivatives are used in embedded systems
sh4 in 1998 (though possible usage via "J2 core" using expired patents)
This means that most are nearly 20 years old or older.
Rust target triples exist for:
m68k: https://doc.rust-lang.org/nightly/rustc/platform-support/m68... and https://doc.rust-lang.org/nightly/rustc/platform-support/m68... both at Tier 3.
(Did not find target triples for the others.)
If you are using these machines, what are you using them for? (Again, genuinely curious)