So since the release of Qt Installer 4.0.0 someone changed files on server and qt doesn't compile for WebAssembly when using QWebSockets anymore. In fact, it tries to add many libraries not used in the project, making it much bigger, that intended.

File missing hen compiling, using Qt += websockets in .pro file:

Qt/5.15.2/wasm_32/qml/Qt/Websocket/libdeclarative_qmlwebsockets.a

I only use websocket from C++, so there is definitly no need for this lib, but even if it was needed, it doesn't exist at this location.

When I resolve this by making a soft link to the correct location of this file, it still crashes due to:

wasm-ld: error: initial memory too small, 17083472 bytes needed

I resolved it by using sed to add '-s TOTAL_MEMORY=33554432' to arguments in generated Makefile. While it helps me compile my project, it doesn't change the fact, that it tries to add many libraries I don't use, when it shouldn't and didn't use to.

Below i show list of all libraries it tries to add to project. It's a very small project and I'm sure 

Qt/5.15.2/wasm_32/plugins/platforms/libqwasm.a Qt/5.15.2/wasm_32/lib/libQt5EventDispatcherSupport.a Qt/5.15.2/wasm_32/lib/libQt5FontDatabaseSupport.a Qt/5.15.2/wasm_32/lib/libqtfreetype.a Qt/5.15.2/wasm_32/lib/libQt5EglSupport.a Qt/5.15.2/wasm_32/plugins/imageformats/libqgif.a Qt/5.15.2/wasm_32/plugins/imageformats/libqicns.a Qt/5.15.2/wasm_32/plugins/imageformats/libqico.a Qt/5.15.2/wasm_32/plugins/imageformats/libqjpeg.a Qt/5.15.2/wasm_32/plugins/imageformats/libqtga.a Qt/5.15.2/wasm_32/plugins/imageformats/libqtiff.a Qt/5.15.2/wasm_32/plugins/imageformats/libqwbmp.a Qt/5.15.2/wasm_32/plugins/imageformats/libqwebp.a Qt/5.15.2/wasm_32/qml/QtQuick/Extras/libqtquickextrasplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Templates.2/libqtquicktemplates2plugin.a Qt/5.15.2/wasm_32/qml/QtGraphicalEffects/libqtgraphicaleffectsplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Controls.2/Imagine/libqtquickcontrols2imaginestyleplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Controls.2/Universal/libqtquickcontrols2universalstyleplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Controls.2/Fusion/libqtquickcontrols2fusionstyleplugin.a Qt/5.15.2/wasm_32/qml/Qt/labs/settings/libqmlsettingsplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Dialogs/Private/libdialogsprivateplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/PrivateWidgets/libwidgetsplugin.a Qt/5.15.2/wasm_32/qml/Qt/labs/calendar/libqtlabscalendarplugin.a Qt/5.15.2/wasm_32/qml/QtGraphicalEffects/private/libqtgraphicaleffectsprivate.a Qt/5.15.2/wasm_32/qml/QtTest/libqmltestplugin.a Qt/5.15.2/wasm_32/lib/libQt5QuickTest.a Qt/5.15.2/wasm_32/lib/libQt5Test.a Qt/5.15.2/wasm_32/qml/QtQuick/Dialogs/libdialogplugin.a Qt/5.15.2/wasm_32/qml/QtQuick.2/libqtquick2plugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Controls.2/Material/libqtquickcontrols2materialstyleplugin.a Qt/5.15.2/wasm_32/qml/QtGamepad/libdeclarative_gamepad.a Qt/5.15.2/wasm_32/lib/libQt5Gamepad.a Qt/5.15.2/wasm_32/qml/QtQuick/XmlListModel/libqmlxmllistmodelplugin.a Qt/5.15.2/wasm_32/lib/libQt5XmlPatterns.a Qt/5.15.2/wasm_32/qml/QtQuick/Controls/libqtquickcontrolsplugin.a Qt/5.15.2/wasm_32/qml/QtSensors/libdeclarative_sensors.a Qt/5.15.2/wasm_32/lib/libQt5Sensors.a Qt/5.15.2/wasm_32/qml/QtWebSockets/libdeclarative_qmlwebsockets.a Qt/5.15.2/wasm_32/lib/libQt5WebSockets.a Qt/5.15.2/wasm_32/qml/Qt/labs/qmlmodels/liblabsmodelsplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Shapes/libqmlshapesplugin.a Qt/5.15.2/wasm_32/lib/libQt5QuickShapes.a Qt/5.15.2/wasm_32/qml/QtQml/libqmlplugin.a Qt/5.15.2/wasm_32/qml/QtQml/Models.2/libmodelsplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Particles.2/libparticlesplugin.a Qt/5.15.2/wasm_32/lib/libQt5QuickParticles.a Qt/5.15.2/wasm_32/qml/QtQuick/Controls/Styles/Flat/libqtquickextrasflatplugin.a Qt/5.15.2/wasm_32/qml/QtWebView/libdeclarative_webview.a Qt/5.15.2/wasm_32/lib/libQt5WebView.a Qt/5.15.2/wasm_32/qml/Qt/WebSockets/libdeclarative_qmlwebsockets.a Qt/5.15.2/wasm_32/qml/QtQuick/Controls.2/libqtquickcontrols2plugin.a Qt/5.15.2/wasm_32/lib/libQt5QuickControls2.a Qt/5.15.2/wasm_32/qml/Qt/labs/platform/libqtlabsplatformplugin.a Qt/5.15.2/wasm_32/lib/libQt5Widgets.a Qt/5.15.2/wasm_32/lib/libQt5QuickTemplates2.a Qt/5.15.2/wasm_32/qml/Qt/labs/folderlistmodel/libqmlfolderlistmodelplugin.a Qt/5.15.2/wasm_32/qml/QtMultimedia/libdeclarative_multimedia.a Qt/5.15.2/wasm_32/lib/libQt5MultimediaQuick.a Qt/5.15.2/wasm_32/qml/QtAudioEngine/libdeclarative_audioengine.a Qt/5.15.2/wasm_32/lib/libQt5Multimedia.a -lopenal Qt/5.15.2/wasm_32/qml/QtQuick/Window.2/libwindowplugin.a Qt/5.15.2/wasm_32/qml/QtQuick/Layouts/libqquicklayoutsplugin.a Qt/5.15.2/wasm_32/lib/libQt5Quick.a Qt/5.15.2/wasm_32/lib/libQt5QmlModels.a Qt/5.15.2/wasm_32/lib/libQt5Gui.a Qt/5.15.2/wasm_32/lib/libqtlibpng.a Qt/5.15.2/wasm_32/lib/libqtharfbuzz.a Qt/5.15.2/wasm_32/qml/QtScxml/libdeclarative_scxml.a Qt/5.15.2/wasm_32/lib/libQt5Scxml.a Qt/5.15.2/wasm_32/lib/libQt5Qml.a Qt/5.15.2/wasm_32/lib/libQt5Network.a Qt/5.15.2/wasm_32/lib/libQt5Core.a Qt/5.15.2/wasm_32/lib/libqtpcre2.a 

I didn't specify that many libs, especially as just few weeks ago i worked fine. Until i deleted Qt and downloaded again... Tried for 5.15.1 and 5.15.2, but I think it may be broken for more bersions.