Philemon is not a side note. It is the New Covenant applied to the most concrete human relationship of the ancient world: master and slave.
Paul does not abolish the legal institution of slavery by decree. He does something more radical. He places Onesimus inside the finished identity of the one new man. The old categories lose their power because a greater reality has already arrived.
Read Philemon as a living snapshot of the covenant transition. The letter was written while the Temple still stood and the old age had not yet fully passed. Inside that overlapping period, Paul already treats the new identity as settled fact. Onesimus is no longer defined by his former status. He is defined by Christ.
The same reality continues. Every believer stands in the same finished position: no longer slave or free, but a brother or sister in the Lord.