It's one of the last QtScript users, and duplicating functionality that QNetworkProxy and/or libproxy already have.
He also said, Qt disabled the use of libproxy back when libproxy linked to Qt4 (clashing with Qt5).
That was probably for reading the KDE proxy settings (!), which nowadays can be done by just using XDG_CONFIG_HOME | ~/.config and not linking to any Qt/KDE code...
The biggest problem is how to test all this...
libproxy does not link against Qt anymore, they use kreadconfig5 and friends to read the values. So using that should be safe.
That's what I wrote on the bug report as well: https://bugreports.qt.io/browse/QTBUG-52252
It's still disabled by default and when enabling it manually there's a huge warning in bold to not do that...