#1028970 complains to stderr about "unknown extended header keyword"

Package:
tar
Source:
tar
Description:
GNU version of the tar archiving utility
Submitter:
Marco d'Itri
Date:
2023-01-15 16:03:09 UTC
Severity:
normal
Tags:
#1028970#5
Date:
2023-01-15 15:47:09 UTC
From:
To:
Emitting these warnings to stderr is bad because it breaks other
software which calls tar and does not expect something on stderr.
Either the warnings are useless, and then they should just be
suppressed, or they are relevant, and then tar should be taught about
whatever a "LIBARCHIVE.xattr.com.apple.lastuseddate" is.

To reproduce:

$ wget https://cpan.metacpan.org/authors/id/M/MR/MRSCOTTY/Crypt-X509-0.55.tar.gz
$ LANG=C tar tvvf Crypt-X509-0.55.tar.gz > /dev/null
tar: Ignoring unknown extended header keyword 'LIBARCHIVE.xattr.com.apple.lastuseddate#PS'
tar: Ignoring unknown extended header keyword 'LIBARCHIVE.xattr.com.apple.lastuseddate#PS'
$
#1028970#10
Date:
2023-01-15 15:59:18 UTC
From:
To:
Control: severity -1 critical
And indeed, now lintian starts emitting errors:

E: libcrypt-x509-perl source: unpack-message-for-orig libcrypt-x509-perl_0.55.orig.tar.gz . tar: Ignoring unknown extended header keyword 'LIBARCHIVE.xattr.com.apple.lastuseddate#PS'
#1028970#17
Date:
2023-01-15 16:01:19 UTC
From:
To:
severity 1028970 wishlist
tag 1028970 wontfix
thanks

Marco, tar doesn't document a standard for stderr. In particular, this
looks like a legit warning in either case. Please bring this up with the
software that is broken by it.