After emerging mail-client/claws-mail-2.7.0 and mail-client/sylpheed-claws-2.6.1 as dependency there is no /usr/lib/pkgconfig/sylpheed-claws.pc as required for most plugins. mail-client/sylpheed-claws should make a link from /usr/lib/pkgconfig/sylpheed-claws.pc to /usr/lib/pkgconfig/claws-mail.pc which does exist. Reproducible: Always
No, you should upgrade to claws-mail-<pluginname> packages, which are using the new claws-mail.pc. In fact, there's a script that does the migration for you - ${PORTDIR}/mail-filter/claws-mail/files/plugins-rebuild.sh. It will first unmerge sylpheed-claws-<pluginname> packages, and merge claws-mail-<pluginname> packages for plugins which you had installed before the migration. As for mail-client/sylpheed-claws-2.6.1, it's a dummy package that can be unmerged at any time.
See comment #1