Is money I earn through Upwork or Deel taxable in Turkey?

If you are a Turkish tax resident, yes — regardless of where the platform or the client sits, and regardless of which country the money lands in first. What changes is the category of income. Project work for multiple clients is generally professional income; full-time work for a single foreign employer through an employer-of-record such as Deel or Remote is generally employment income. The two are taxed under different rules and different reliefs.