QtWebEngine 5.15.3 is scheduled for its final update of Chromium to be based on Chrome 87. This versions makes node.js mandatory for building the devtools, and we haven't found a way to make it optional again. 

Also it appears the node.js version 8.10 installed on Ubuntu 18.04 doesn't compile correctly. Though it is inconsistent, see http://coin.intra.qt.io/coin/integration/qt/qtwebengine/tasks/web_qt_qtwebengine_1607619254030 with some platforms managing to build but not link, while others seem to not be able to parse "async import".

I believe the simplest solution would be to use a more modern node.js version, either by downloading and installing official packages like we do for Windows, or by using a PPA like https://deb.nodesource.com/node_14.x/.

Note SLES also is missing node.js, and at least downloading packages could also work for it.