That’s… not actually true. All of those words mean different things. ‘Peasant’ is a class of farmers. A ‘serf’ is a specific subset thereof - some peasants were actually slaves, others were free, and some were serfs, which was a strange state of semi-enslavement.
‘Laborer’ is really anyone who does work. It’s implied that it’s largely physical work, but that could be anything from farming to construction to assembly-line work.
‘Villager’ is an occupant of a village. Even if you’re using it to describe a social class, it still doesn’t imply much about what work they do - a villager could be a farmer, a shopkeeper, or a blacksmith.
As far as I know, ‘countryman’ just means ‘resident of the same country’.
‘Poor person’ describes socio-economic status, but again, it doesn’t say anything about why the person is poor or what work they do.
‘Servant’ is a job description. Servants are paid, and are free to stop being servants if they wish, or to go and work as servants for someone else. They might do work similar to some slaves, but the concepts are very much distinct.
If you really need synonyms for slave, ‘chattel’ and possibly ‘bondservant’ might do. But personally I think ‘slave’ is a good word for that. It’s a harsh sound for a harsh concept. If there were still slaves and they wanted to be called ‘enslaved persons’, I would think that a good reason to do so… except that if that were the case, I’d be more concerned with the part where we had people owning other people.