I recently configured a laptop with Ubuntu 18 LTS and installed Audacity 2.4.1 from the Ubuntu Software application.
This install method uses snap.
Unfortunately the help page for installing ladspa plugins did not help for this install method.
After sometime I got it to work so I thought I would share here. There may be a better method and I only tried ladspa plugins.
to install a ladspa plugin if audacity was installed via snap:
copy the plugin .so file to
start or re-start audacity.
Now use the Add/remove plugins menu as usual.