Macdown was a very helpful alternative to Mou when Chen Luo abandonned it, but it seems that uranusjr abandonned Macdown too.
Though, it's still lightweight, functional, with good feature (themable, fully markdown support , support of TOC, shell plugin, neat keystrokes, multiple views, tab bar, …), which made it really a complete tool, the market of markdown editors really evolved, and neither paid nor free or FOSS apps are now competing.
Some go to be notes apps (libke Bear, MWeb, which is one of my favorites, but quite expensive, FSNotes, or the open source Joplin, Notable Boostnote). Others become diary on journal tools (MWeb again, or but-now-only-subscription model DayOne), or try to be full writing tool like the veteran iA Writer, ByWord, or the complete-but-now-only-subscription model Ulysses, but also the new FOSS complete Zettlr.
It will end that only nostalgic will use it, since it become not compatible with the last evolution on the OS.
Is use to love but don't open it anymore (except for comparison with other tools).