QGIS shortcut to transparency

Credits to @Kazuhito from his answer https://gis.stackexchange.com/a/284797/99589

Try the "Opacity slider" via RMC > Properties > Legend > Embedded Widgets in Legend > Available widgets

and edit the transparency

This is more a workaround but still can be useful:

Transparency/Opacity can be a tailored globally (all layers etc.) or individually by layer.

This way is by layer:

Layer Styling [Shortcut] is F7 in QGIS.

This can be docked in QGIS for quick access. This then allows the user to pick the slider for any of the layers in the canvas including images/raster's.

This screenshot shows where layer styling is available [or use F7 on the keyboard to bring it up or remove the panel]

Windows 10 Pro (QGIS 3.14 used here)