From the start of the industrial revolution through about the 1960s, the principle mechanisms were division of labour and application of more energy per worker. There was some automation, but it was largely at a very simple scale -- Jaquard's loom, duplicating patterns on lathes and machining systems, dies (a simple mode for reproducing complex shapes).
But for the most part, what wasn't being substituted for was the worker's skill.
Since the 1960s or so, that's begun changing. Soup-to-nuts factories produce goods with little or no worker intervention. CAD-CAM systems translate design directly to machining.
Automation, in short, replaces the mind of the worker, rather than the muscle.
This also calls to mind the question of just what it is that labour is. I'd consider it a compound factor of production:
1. A prime mover.
2. A complex manipulation system.
3. Skill and processing knowledge.
4. A control system through sensing, processing, feedback, and resulting process management inputs.
Labour-as-muscle has been subject to replacing since the time of draught animals, water, and wind power, though at a vastly greater rate with steam, electrical, and internal-combustion power. The result has largely been an amplification of labour via energy inputs.
Substituting for the information-processing capabilities of labour is another matter. It does mean, however, that increasing amounts of skilled work might be replaceable with a much less-skilled technician. I've seen this trend in the IT world over the past several decades, though there remain points at which greater skills are still in demand.
This makes the viability of coerced labour, including the coerced labour of sweatshops and workhouses, as well as actual prison slave labour, for complex goods, far more viable.
Let that spin your moral compass for a bit.
It's not so easy to draw firm conclusions from Roman society, because the historical record of that is a little thin. The Nazis employed slave labor, but they had a lot of trouble with sabotage and intentionally shoddy work. Game of Thrones is entirely fiction, and is zero evidence as an "example" of slavery.