Do I need a custom domain for my link in bio?

It's the single biggest thing you can do to keep a link from getting flagged, because a domain with its own history isn't dragged down by a shared one. The catch is the price. On Linktree a custom domain is a paid plan, roughly $24 to $35 a month, so the fix for the flagging problem sits behind a paywall.

itsmylinks skips that. You're on an established, trusted domain from the start, at no cost, so you get the reputation benefit without renting it by the month.