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Zelensky is the first one to officially bring up demilitarisation of the Black Sea
(english.elpais.com)
After the Russian invasion of Ukraine on 24 February 2022, the Ukrainian government appealed to Turkey to exercise its authority under the Montreux Convention to limit the transit of Russian warships from the Mediterranean to the Black Sea. At least six Russian warships and a submarine had crossed the Turkish straits in February. After initial reluctance, attributed to the country's close ties with both Russia and Ukraine, Turkish Foreign Minister Mevlüt Çavuşoğlu announced on 27 February that his government would legally recognise the Russian invasion as a "war", which provides grounds for implementing the convention with respect to military vessels. This blockage of naval vessels also applies to NATO powers who cannot now move their vessels from the Mediterranean to the Black Sea. However, Çavuşoğlu reiterated that pursuant to the terms of the agreement, Turkey cannot block Russian warships based in the Black Sea from returning to their registered base.
Two Russian Federation ships (the missile cruiser Varyag, the flagship of the Russian Pacific Fleet, and Admiral Tributs, a large anti-submarine destroyer) waited in the Mediterranean seeking to enter the Black Sea for nine months. In October 2022, they were refused permission and left the Mediterranean Sea through the Suez Canal.
On 2 January 2024 Turkey refused passage through the strait to two minehunters donated by the British Royal Navy to the Ukrainian Navy pursuant to the Convention.
for more information ☞ https://en.wikipedia.org/wiki/Montreux_Convention_Regarding_the_Regime_of_the_Straits