Hey, thanks so much for the response.
My understanding from L4T3NCY's post is that Showdown is the one exception to the 'RGMs are difficult to support cross-patch' rule because it's just a small variation of ARAM, so nothing should break from patch to patch. He said it would be "effectively zero upkeep".. was he wrong?
I know there's the other issue of enabling RGMs that are currently live for matchmaking in customs (something to do with matchmade game modes and custom games running on different server tech - L4T3NCY refers to the problem towards the end of his post) but that's not at all what the Ask Riot article discusses.
I'm not sure exactly what data or analysis he was operating from, so I can't speak directly to that. I can say that I've been a part of some recent-ish evaluations of what it would take to bring RGMs (including Showdown) to customs, and the short answer was "a lot". Codebases also change a lot in 3+ years, particularly in cases like League where there's new stuff every two weeks, so it may have become harder over time.