The Designers themselves are pretty stable and so I won't be adding new features to them.
New versions of WebbotLib itself are also fairly infrequent as it now seems to support most AVRs that people use.
That just leaves supporting new devices which previously required a new release of WebbotLib each time. It also means writing each device for WebbotLib as well as WebbotLib Studio since Studio has its own 'more tuned' version of WebbotLib underneath. The benefit of Studio is that people can choose whether to use the new device or not. If you decide to use it then it only needs to download the new code for that device (a few Kb say) rather than the whole of WebbotLib.
So I guess the answer to your question is - yes it will be minimally maintained - but I would prefer to only add new devices to WebbotLib Studio.