I think an Imperial Knight sub-forum would do good. One thing I've noticed is that the Imperial Knights get stuffed into the Adeptus Mechanicus category. I think this is a little odd for a few reasons.
1. It appears that as a rule, when an army gets its own Codex Supplement or Codex, a la Defenders of Terra, Codex: Dark Angels, Codex: Deathwatch, it gets its own subforum. I think this is a fairly reasonable way of doing things. I think it's a little odd that Militarum Tempestus don't have their own subforum, but given that Militarum Tempestus are an anemic Codex and their units are even in the Guard Codex, it's not that unreasonable.
Codex: Imperial Knights is definitely a thing. The Imperial Knights have a standalone Codex with their own force organisation, warlord traits and relics. There are even kind of two editions (the first one, featuring only the Knight Errant and Paladin, and the newer one featuring all the Citadel Knight variants and new wargear). Unlike Militarum Tempestus, whose rules are also rolled into Codex: Astra Militarum and they don't even get their own relics, Imperial Knight rules and wargear are found solely in their Codex. And even though they're often a supplementary force, they are used in MANY different army configurations, not just with Mechanicus. I also get why Cult Mechanicus and Skitarii might be a single sub-forum, as they're so alike and coordinate so well, and players often use them together, but I don't think this is quite the same for knights.
2. It's a little odd from the lore perspective. Imperial Knights are derived exclusively from noble houses. Even though many noble houses align themselves with the Mechanicus, they still operate independently much of the time. Their fighting style and organisation is vastly removed from the cold, utilitarian, cyborg style of Mechanicus. Knight houses are flamboyant, over the top and grand with their own institutional culture. Even when they align themselves to the Mechanicum (and not always), they still retain their noble trappings. They just repaint their knights. On the tabletop, I know knights see a lot of use with the Mechanicum as allies and via the War Convocation, but knights are a mainstay in many different armies, and not just Imperial Armies but also Chaos armies with Renegade Knights rules coming with the Traitor's Hate supplement.
3. Knights are so far removed from Mechanicus that I think they deserve a subforum. The Imperial Knights are unique in being an entire army of super-heavy models. Their Codex is packed with rich history, culture, heraldry and flamboyance. They are very, very different from the aesthetic of the Mechanicus. They are not even in the same branch of the Imperium, the way Cult Mechanicus and Skitarii are. They're used as support elements for many armies, not exclusively Mechanicus and having them in their own subforum means that people who want to see how to use multiple knights fielded can learn more, easily. Imperial Knights are an Imperium player's first super-heavy nowadays, so they're a big step for a lot of players.
4. Imperial Knights often feature bitchin' paintjobs and fantastic conversions as a consequence of being very different models. The subforums are often used for faction-specific modelling and painting advice or showcases and Imperial Knights make amazing showcase and centrepiece projects.
5. I guess what i'm trying to get at here is that there aren't a lot of players who field solely Imperial Knight armies. But I think Imperial Knights are different and important enough that they should have a sub-forum for discussion.