/usr/include/GNUstep/NGImap4/ is in libsope-dev 2.2.17-1build2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/GNUstep/NGImap4/NGImap4.h
/usr/include/GNUstep/NGImap4/NGImap4Client.h
/usr/include/GNUstep/NGImap4/NGImap4Connection.h
/usr/include/GNUstep/NGImap4/NGImap4ConnectionManager.h
/usr/include/GNUstep/NGImap4/NGImap4Context.h
/usr/include/GNUstep/NGImap4/NGImap4DataSource.h
/usr/include/GNUstep/NGImap4/NGImap4Envelope.h
/usr/include/GNUstep/NGImap4/NGImap4EnvelopeAddress.h
/usr/include/GNUstep/NGImap4/NGImap4FileManager.h
/usr/include/GNUstep/NGImap4/NGImap4Folder.h
/usr/include/GNUstep/NGImap4/NGImap4MailboxInfo.h
/usr/include/GNUstep/NGImap4/NGImap4Message.h
/usr/include/GNUstep/NGImap4/NGImap4ResponseParser.h
/usr/include/GNUstep/NGImap4/NGImap4ServerRoot.h
/usr/include/GNUstep/NGImap4/NGImap4Support.h
/usr/include/GNUstep/NGImap4/NGSieveClient.h
/usr/include/GNUstep/NGImap4/NSString+Imap4.h