Everyone does, including people whose passport normally gets them into Türkiye without one. Every registered participant has to go through the COP31 e-Visa platform, and those who are visa-exempt receive a Visa Exemption Certificate instead of a visa.
This is the rule that catches people out, because it inverts the usual logic. Türkiye has authorised a free electronic COP31 Visa for registered participants, and the presidency states that all of them must apply "regardless of their visa-requirement status". If you are exempt, you still apply, and what you get back is a certificate confirming the exemption. The presidency is blunt about what happens if you skip it: failing to obtain the document before travelling may result in delays or refusal of entry.
Already holding a Turkish visa that covers November does not let you off either. The presidency says holders of a valid visa covering the COP31 period still need the COP31 Visa or the exemption certificate, because that is what registers you at the border as an approved participant.
Two other conditions apply at entry and have nothing to do with the COP. Your passport must be valid for at least 90 days after the date you arrive, which is a longer margin than many countries ask for and is worth checking now rather than in October. And Turkish regulations require every foreign visitor to hold valid travel insurance, which border officers may ask to see.