The rust-sequoia-openpgp source package is in good shape.

But, due to #1001251 librust-sequoia-openpgp+crypto-nettle-dev was
renamed to rust-sequoia-openpgp+nettle-dev.

As a result, librust-sequoia-openpgp+crypto-nettle-dev 1.3.0-4 lingers
in unstable, even though the source package is at 1.8.0-1.  Removing
the old version of librust-sequoia-openpgp+crypto-nettle-dev should be
sufficient to leave the archive in an acceptable state, as the
Provides: entries can satisfy what needs to be satisfied.

Thanks for maintaining the unstable archive in debian!  Sorry for the
additional hassle.

This package seems to have vanished, so I am manually closing this bug now.

   Thorsten