I know the 3ed material you are basing this off of, but I'm looking at it with just a 5e eye right now.
Primary race
Half-living: I'm tryng to decided if this is a big detriment or not so much. In the 3ed material there was a bunch of arcane spells that could repair, so it shifted the burden from the cleric to the wizard. But 5e has both a lot more self-healing with HD, and to bring back someone unconcious you start at 0, so you only need to roll a 2 or higher to wake. (A 1 halved then rounded down will give 0.)
Composite Plating. Dex and Con are nice scores and you start with +2 Con and possibly +1 Dex. This is a decent boost for all light armor wearers (even with studded leather at Con 14 which is dirt simple with +2 Con), saves wizards and sorcerers from using a spell known and a slot for mage armor. In some ways I'd rather see a barksin-like effect to represent the armored components. "If your AC would be less than 14 after all modifiers it is instead 14."
Subraces
Scout
"Small" had it's own set of advantages back then which 5e does not have. This is weaker than either of the other two. Perhaps grant the natural weapon finesse.
Commando
I've never seen an "ability score of choice" on a race with subraces. Usually that's one of the determinations for which subclass to pick.
Charger
My first thought was this was overpowered. I was using dwarf, the only other +2/+2 race, and it has similar options (poison advantage/resist, 25' move, etc.) But with Lumbering I can't quite say it. It's one of the more powerful races, and very targeted towards doing one thing, but that's not the same as overpowered.
Feats
Jaws of Death - what does the bite bring over Natural attacks? Not sure this brings you enough to spend a half-feat on.
Overload Metabolism - need clarification. Is "plus additional hit
points equal to your maximum number of hit dice" the same as "level"? So a Fighter 10 had 10d10 HD and adds 10 HP to the healing? Or is it the actual HD that are added. If it's the former, just say level. If it's the latter, OMG this is broken.
Augmented Slam - Damage doesn't scale making that part become irrelevant soon. The second part comes up about 5% of attacks, and only if the attacks are against a construct which is not a common foe type, and if it fails a save. That sounds like it comes into play maybe once per campaign. That's not enough at all for a feat.
Spiked Body - doesn't scale, because useless very quickly.
Aspect of Autonomy - umm, will need to go into depth on this one. Poison Breath and Slow seem Iron Golem like, not warforged like. Berserk is ... well, especially with Second Slam for more attacks could be doing a lot of them, but without options on who. Is that the type of thing that players would want?
Subclasses
Archetype of the Juggernaut
Siege Craft - hmm, unarmed gets up to d4+6 "base" damage, average 10.5. That's significantly better that all two handed weapons. Eh, I'm fine with that, that's only at later levels and the few extra points per hit aren't a big deal. They also qualify for Great Weapon Master.
Oh, and a Warforged (Scout) now gets disadvantage on all unarmed attacks. They are small, and that's what Heavy does. Seems odd to make a racial subclass replacement and then exclude one of your subclasses from it.
You know, this feature makes me revise my thoughts on the Second Slam feat. It's fine with normal d4/d6. Like dual wielding but your other hand can hold a shield or something. But when each attack is now more than a greatsword that feat becomes overpowered. Let's see at 12th level (two ASI, Great Weapon Master, Second Slam - gotta love fighter extra ASIs) you could be make four attacks for d4 + 4 (prof) + 5 (str) + 10 (GWM) +2 duelist fighting style, averaging 23.5 per hit (disregarding criticals) or 94 if they all hit and holding a shield in the other hand. A normal fighter with GWM and a greatsword would have 3 attacks for 2d6 + 5 (str) + 10 (GWM) averaging 22 per hit or 66 if they all hit. So this is doing more that 50% more, and gets +2 AC for the shield.
If you get rid of the Heavy it takes away GWM and puts in in the area of dual weapon fighting. It still does more damage than that and has a better boost to armor class, but it's not outrageous. Oh, and suddenly Scouts can use the subclass.
Path of the Colossus
Heavy buttressing - likely for speed of play, every other type of thing like this just refers to creature size. Perhaps just "Can't be XX unless Large or larger".
Absolute Resilience - is there an official order to apply Resistance and damage reduction? That "5" might really be a 10 in most casts.
Path of the Vengeful Steel
The continual reducing of Int, Wis, and Chr seem like it would apply to a very limited set of players - those who like playing stupid, unwise characters with little force fo personality who are charmed, held or dominated often at high levels of play. I'm not sure that's even a niche market.
Natural Momentum - Seems to assume that your first class has CON as a proficient save, which isn't always the case. Either that or is a lot more powerful barbarian feature if you're not a barbarian first, which makes little sense.
Ferocious Power - you have resistance to B/P/S AND you reduce it by 3 (6?). At 6th level? Unlike Colossus' Absolute Resistance at 14th, this seems too good at the level you get it. Also, can you cast that Dispel Magic only at yourself, or at anything? Fluff seems to say the former, but the mechanics don't say that.
