The only non-UN states that undoubtedly meet the standard of statehood are the Cook Islands and Niue, who have had their "full treaty-making capacity" recognised by the United Nations Secretariat.
[1][2] The Holy See (Vatican City) is also widely recognised as being able to legitimately ratify treaties, and has been granted non-member observer state status by the UN General Assembly.
Ratifications performed by other states with more limited recognition—such as Abkhazia, the Republic of Artsakh (Nagorno-Karabakh), the Republic of China (Taiwan), Kosovo, Northern Cyprus, the Sahrawi Arab Democratic Republic (Western Sahara), Somaliland, South Ossetia, and Transnistria—have usually not been recognised by treaty depositaries as states that can ratify treaties, although there are some exceptions to this general rule.
In some cases, such states are subsumed into an existing state, as when East Germany merged into the Federal Republic of Germany, and when Zanzibar merged into Tanzania (at first United Republic of Tanganyika and Zanzibar).
[c] For purposes of the numbers in this list, only ratifications, accessions, or successions of currently existing states are considered.
If supranational or other international organizations ratify the treaty, the total number of ratifications may exceed 198.