I have no idea how you concluded that Factor's not actively developed. Factor 0.95 was just released on August 17th, and if you look at the GitHub history page, you'll see that there are a lot of commits landing on a regular basis. Most aren't done by Slava, as he's handed off the project to Doug Coleman (erg) and John Benediktsson (mrjbq7), but the language is continuing to see major improvements—some amazing. (E.g., global-value numbering, which can result in massive speed boosts throughout the entire code-base by factoring out common expressions into a single location, just landed a few weeks ago.)
While you're right that there's a fair amount of code in
unmaintained, there's a much greater volume of code that's in the core libraries—and in fact, the majority of code in
unmaintained amounts to thought-experiments or code that no longer represents and ideal way to work in Factor. I.e., in at least some circumstances, it'd be better called
As far as other stack-based languages, there are some in active development, but none have the traction Factor has, and the only concatenative language I know with wide distribution other than Forth is PostScript—probably not what you want to write your next project in. Unless you're unhappy with something Factor's offering, I think it's your best bet. The community's friendly and active, and I'm sure would be thrilled to answer any questions you have or to accept any improvements you have in mind.