So, you are basically looking for abandoned games with active mods..? Or just low amount of runners games?
Erh, well like @komrade said, you can use the filters on the games page.
There's a good chance this ^ will link you to a quiet leaderboard.
"www.speedrun.com/randomgame" Whoa. New homepage.
I like @6oliath's comment, but that does not really get at what @Diewott1is asking. @Komrade's answer is better, but the problem I have with that is that they start with 0 runs. Games, with several of them having a nice notice on them saying that they are being tracked on a different site like Donkey Kong Country. Also with 0 runs I can really only think of them as placeholders. https://www.speedrun.com/games#sorting=runCount&page=33&direction_sort=off gets you to games with at least 1 run. But again that does not distinguish between active and dead games, since some games have a run or two, but are actively being worked on. Or you can have a game that has historically had a lot of activity but is now pretty dead like https://www.speedrun.com/finaldoom#TNT_Evilution . So as of right now in this thread there is no good answer.
Something like getting a list of games where the last submitted run was over 6 months ago might work.
if you look for zx81 games (very abandoned games :D) : 3D Monster Maze : https://www.speedrun.com/3D_Monster_Maze Labyrinth : https://www.speedrun.com/labyrinth_zx81
Basically a thread where anyone can advertise games. Won't be as useful as one might think. People are already free, more or less, to make a post on the board saying "Hey! Come check out this game for speedrunning." The thing is, most people are already expending a good amount of time on whatever it is they are playing. Picking up a new game and getting a decent run might require months of effort, which is hard to fit into your schedule when you are playing another game for months. Additionally, most of the people speedrunning are not really hanging out on these forums.
IMO, it's better to give suggestions to people who are actively asking on the forums for some games to run. They'll generally give you their platform range and tastes, etc. If a game you know of is a good fit, then by all means. Heck, I refer people just generally to my profile if what they are seeking describes a good portion of the games I've played, and that attracts some people to leaderboards I mod that could more runners. I'd rather see a single thread where people looking for suggestions can ask, but I imagine since a lot of newish speedrunners generally ask, they aren't going to see the specific thread for it and will start new ones anyways.
Oh.
You're looking for something much more narrow.
Good luck.
Also you're wasting your time trying to grow any community by begging people to check the game out on a forum.
You can check out existing communities like Obscure Speedruns Club.
You'll learn to love Komrade, although his input is often as useful as a lobotomy. <3 ya Kom
Just the reality of it. No amount of shedding light on any game will cause the community to have a great surge of interest. Best you can do is create a leaderboard, create a discord server, and in 6+ months you might find another runner.
Alternative: Create short meme categories that anybody can pick up in 5 minutes. Won't gain any real runners but it would "grow the community".
Basically you can't force people to like or play a game. And if the leaderboard is already dead, safe bet it'll stay that way for a long time.
Making guides for the game to make it easier to pick up and actively streaming runs of it also helps.