I haven’t found anything already posted about this problem.
https://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins shows an entry for Ramp Panning whose Download link is http://old.audacityteam.org/nyquist/panramp.ny . That URL produces:
Hmm. We’re having trouble finding that site.
We can’t connect to the server at old.audacityteam.org.
None of the other listed Effects plugins appear to do the same thing. If there’s a way to achieve a smooth left-to-right pan without the Ramp Panning plugin, please inform.