sorry for the duplicate! thanks for the quick response.
a third option, which i pursued, is simply rebuilding mosquitto
from source (apt-get source + debuild). this properly linked
against libdlt.so.3:
[qgp](0) $ ldd $(command -v mosquitto)
linux-vdso.so.1 (0x00007f6b289f7000)
libwrap.so.0 => /usr/lib/x86_64-linux-gnu/libwrap.so.0
(0x00007f6b28976000)
libsystemd.so.0 => /usr/lib/x86_64-linux-gnu/libsystemd.so.0
(0x00007f6b2885d000)
libdlt.so.3 => /usr/lib/x86_64-linux-gnu/libdlt.so.3
(0x00007f6b28820000)
libssl.so.3 => /usr/lib/x86_64-linux-gnu/libssl.so.3
(0x00007f6b28713000)
libm.so.6 => /usr/lib/x86_64-linux-gnu/libm.so.6 (0x00007f6b2861d000)
libwebsockets.so.19 => /usr/lib/x86_64-linux-gnu/libwebsockets.so.19
(0x00007f6b2859b000)
libcrypto.so.3 => /usr/lib/x86_64-linux-gnu/libcrypto.so.3
(0x00007f6b27e00000)
libc.so.6 => /usr/lib/x86_64-linux-gnu/libc.so.6 (0x00007f6b27c0a000)
/lib64/ld-linux-x86-64.so.2 (0x00007f6b289f9000)
libcap.so.2 => /usr/lib/x86_64-linux-gnu/libcap.so.2
(0x00007f6b2858f000)
libz.so.1 => /usr/lib/x86_64-linux-gnu/libz.so.1 (0x00007f6b2856f000)
libzstd.so.1 => /usr/lib/x86_64-linux-gnu/libzstd.so.1
(0x00007f6b284a3000)
[qgp](0) $
