I don’t think there’s been a lot in the way of new development but I don’t think Lemur is dead. It looks like the folks at Liine need to update the SSL certificate, but the site is still there.
I do use Lemur, though I greatly prefer the design and feel of the components in Touch OSC. On the other hand, Lemur seems to have a more robust library of user contributed templates. Though, if you’re a Reaper user, I find the “LogicPad” template in Touch OSC to be beyond amazing.
I can also confirm that the editing software is still working under Mac OS Catalina for both Lemur and Touch OSC.
I think both Lemur and Touch OSC are worth having in your quiver, though.