There is also this Nyquist plug-in that seems to match your description: https://forum.audacityteam.org/t/simple-plug-in-for-creating-partial-fades/16055/1
See here for how to install plug-ins: http://audacityteam.org/download/plugins
There is also this Nyquist plug-in that seems to match your description: https://forum.audacityteam.org/t/simple-plug-in-for-creating-partial-fades/16055/1
See here for how to install plug-ins: http://audacityteam.org/download/plugins