Would we still use slaves if technology never went further ahead?

Slavery exists as it is, but it seems somewhat doubtful for acceptance of slavery to grow, since that would depress wages and increase unemployment, which breeds instability. Having said that, the reverse can happen; slavery leads to a decrease in technological progress since inefficiency is acceptable when labor costs are low.

We still have slaves. Wage slaves. People who work for an unlivable wage. Millions of them!

Probably. They might not be called slaves though and they would be paid.

Have you ever seen Upstairs Downstairs or Downton Abbey.
If technological development stalls and inequality keeps rising perhaps we will see the return of domestic service which was pretty close to slavery.

But historically it has been the other way about: having slaves means not needing to develop technology because someone else is doing the work and that someone else is a renewable resource.

We'll never actually know, but regardless, industrialization was worse for the planet as a whole. Slaves and industrialization were mostly products of capitalism.

Yes. Industrialization is the only reason the slightest serious consideration was ever given to abolishing it.

Seeing as slavery was abolished a long time ago, I doubt it very much. What people would use was paid labour a lot more. Sure.

Perhaps not, because automation can be used. Companies can use robotics now.

There still is slavery

Slaves are still used now in many places I believe. Sexual slavery I think is the most common across the world though.