LADSPA v1 is ancient and fundamentally quite broken, and that's what they dropped. Audacity 4 ships with LV2 (i.e. LADSPA v2) support.

FWIW I don't think I've used a v1 LADSPA plugin for a decade.

it might be ancient, but it's also very simple. it's relatively easy to import a single header and write a working plug-in, which makes it attractive for enthusiasts.