This code is licensed under LGPLv3 or LGPLv2.1.
That could be seen on http://code.qt.io/cgit/qt/qtwebsockets.git/tree/README.md?h=5.9.0#n37 and https://github.com/qt/qtwebsockets#license , for example.
Code lists only LGPLv3 and GPLv2/GPLv3+, does not list LGPLv2.1, and states:
** Copyright (C) 2016 Kurt Pattyn <email@example.com>.
** Contact: https:**
** This file is part of the QtWebSockets module of the Qt Toolkit.
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https:** information use the contact form at https:**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 3 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL3 included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 3 requirements
** will be met: https:**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 2.0 or (at your option) the GNU General
** Public license version 3 or any later version approved by the KDE Free
** Qt Foundation. The licenses are as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
** included in the packaging of this file. Please review the following
** information to ensure the GNU General Public License requirements will
** be met: https:** https:**
Also, the LGPLv2.1 license text is not included in the repo as it should be per the Readme.