Following statements copied from https://cygwin.com/licensing.html (30.11.2019):
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What are the licensing terms?

Most of the tools are covered by the GNU GPL, some are public domain, and others have a X11 style license. To cover the GNU GPL requirements, the basic rule is if you give out any binaries, you must also make the source available. For the full details, be sure to read the text of the GNU General Public License (GPL).
The Cygwin™ API library found in the winsup subdirectory of the source code is covered by the GNU Lesser General Public License (LGPL) version 3 or later. For details of the requirements of LGPLv3, please read the GNU Lesser General Public License (LGPL).
For more information on the GPL see the GPL FAQ.

Cygwin™ Linking Exception
As a special exception, the copyright holders of the Cygwin library grant you additional permission to link libcygwin.a, crt0.o, and gcrt0.o with independent modules to produce an executable, and to convey the resulting executable under terms of your choice, without any need to comply with the conditions of LGPLv3 section 4. An independent module is a module which is not itself based on the Cygwin library.

Does Cygwin™ have an ECCN number?
No. Cygwin source and binary are made publicly available and free of charge to download so Cygwin is provided under TSU/TSPA exemption. As a result, Cygwin does not require an ECCN number.