On x86_64-unknown-netbsd9.3, running Autoconf's test suite consistently produces this failure:

Failed tests:
 NUM: FILE-NAME:LINE     TEST-GROUP-NAME
 270: torture.at:913     Substitute and define special characters

This test checks whether it is possible to pass a string through AC_SUBST that includes a byte with every possible value except 0x00.

The log for the testsuite says

configure: creating ./config.status
config.status: creating Foo
config.status: creating Zardoz
config.status: creating config.h
./torture.at:987: cat Foo
./torture.at:1000: cmp allowed-chars Zardoz
--- /dev/null   2023-12-19 14:19:58.701932966 -0500
+++ /net/u/1/z/zackw/autoconf-2.72d.16-9800e-dirty/tests/testsuite.dir/at-groups
/270/stdout     2023-12-19 14:19:58.910181000 -0500
@@ -0,0 +1 @@
+allowed-chars Zardoz differ: char 256, line 2
./torture.at:1000: exit code was 1, expected 0

Comparing hex dumps of the files "allowed-chars" and "Zardoz" shows that the problem is that an extra byte (with value 0x81) has been inserted before each byte with a value in the range 0x81 through 0x8b.  The extra bytes are not present in the value of @zardoz@ recorded in config.status, but they are present in the temporary file `subs1.awk` used by config.status (to see this file run `./config.status -d` and then look in the directory named confXXXXXX, where XXXXXX is some random sequence of alphanumerics).  Thus, this appears to be a bug in the handling of here documents in this system's /bin/sh.  However, the obvious cut-down version of the code in config.status does not show the bug:

$ printf 'xxd <<\\EOF\n\x80\x81\x82\x83\x84\x85\x86\x87\n\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\nEOF\n' > test.sh
$ /bin/sh test.sh
00000000: 8081 8283 8485 8687 0a88 898a 8b8c 8d8e  ................
00000010: 8f0a                                     ..

We need to pin down exactly what the bug is before we can do anything about it.

If you encounter a failure in this test on any platform besides -unknown-netbsd9, please post a comment here.  Please attach a tarball of the directory tests/testsuite.dir/270 (note: the number may be different if you are testing a version of Autoconf other than 2.72).