If I equip Jhin with Svellsongur, does the deflect also get copied?

Yes, equipping Svellsongur to Jhin, Murderous Artist gives Jhin Deflect 2 — increased from his printed Deflect 1.

Here's why: Svellsongur copies the unit's printed text to its own effect text. Jhin, Murderous Artist has [Deflect] printed on him (Deflect 1 per Rule 809.1.b.3). Svellsongur's effect text gains that same [Deflect], which grants the unit an additional Deflect 1. Per Rule 809.2, Deflect values from multiple sources sum together, so Jhin ends up with Deflect 2.

However, Svellsongur itself does not gain Deflect (FAQ #7465). If an opponent targets the gear directly (e.g., with Salvage), they pay no Deflect cost — that protection only applies to spells/abilities that choose Jhin himself.