The states that have ratified the Convention but have since denounced it and withdrawn from the agreement are Bulgaria, Hungary, Mongolia, and Poland.
On the other hand, Poland's gov't website says they are (still) in...
The PAA bears responsibility for the performance by Poland as the contracting party of below treaties, conventions and international agreements: [...]
Convention on Early Notification of a Nuclear Accident, done at Vienna on 26 September 1986 (Date of ratification by Poland: 24.2.1988)
Image cropped to remote lots of other treaties:
So, did Poland really withdraw, i.e. fully denounce the Convention on Early Notification of a Nuclear Accident? (And if so, when?)
Hungary's website is less clear on dates, but also says:
Hungary is a contracting party to the following international conventions on the peaceful, safe and secured use of atomic energy: [...]
Convention on Early Notification of a Nuclear Accident (ENC)
So the Wikipedia claim seems in doubt on that as well.