is proponence a word?

Yes. It is the noun for proponent, but your sentence could be simplified.

Opposition is the antonym of proponence.

Frank stresses Aristotle's opposition to slavery and the slave trade.

Proponence is not commonly used...but it is correct...

You might re-word and say that Frank is an advocate, or endorses Aristotle's abstention from slave activities.

I am not sure if your intent is to say that Aristotle favored in abstaining in the slaves' actual activities, (such as working in the fields, for example) or if he favored abstaining from the activity of slaves, period, as in not having anything to do with them, including owning them.

Yes.

I googled it and found it in a sentence:

"opposition to or proponence of slavery"

"His proponence of the Colonial"

I can't think of any other better word right now.