The header of source files in the Qt WebSockets modules state:

** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.

But there is no LGPL_EXCEPTION.txt file included in the 5.4.0 release.

There is a LICENSE.GPLv2 file but no mention of it any source file header or README.md.

The licensing section of README.md states simply that "This code is licensed under LGPL v2.1" but all the source file headers state that "... this file may be used under the terms of the GNU Lesser General Public License version 2.1 or version 3 ..."