GlassIt VSC is a popular Windows-only transparency extension for VSCode. GlassIt Linux expands that functionality to include standard Linux window managers such as GNOME.
GlassIt Linux requires wmctrl (to get the window ID), xprop (to set transparency), and bash (to run the transparency commands) installed on your system. These come standard with most Linux distributions, but just in case, you can install them with:
sudo apt install -y wmctrl x11-utils bash
Install the extension
Restart any single VSCode window
Opacity defaults to 97%, but can be set with the setting Glassit-linux: Opacity in VSCode's settings window. A restart of any single window is required for this setting to take effect.