The claim that the New York Supreme Court said over 800,000 illegal aliens have been voting illegally in New York elections is false because the court struck down a law that would have allowed noncitizens (including legal noncitizens) to vote, and there is no evidence that illegal immigrants have been voting illegally in New York elections.
NY Supreme Court says over 800,000 illegal aliens have been voting illegally in NY elections.

PolitiFact - Do ‘illegal immigrants now have the right to vote in New York’? No, that’s False.
Source: politifact.com
Key Evidence
The New York State Supreme Court struck down the noncitizen voting law before it took effect.
reuters.comreuters.comNew York City law allowing non-citizens to vote struck down by court
nytimes.comnytimes.comNew York City’s Noncitizen Voting Law Is Struck Down+1 more
The estimated 800,000 figure refers to legal noncitizens who would have been eligible under the proposed law, not illegal voters.
politifact.compolitifact.comDo ‘illegal immigrants now have the right to vote in New York’? No, that’s False
poynter.orgpoynter.orgSean Hannity is wrong. ‘Illegal immigrants’ cannot vote in New York City’s local...
Fact-checkers explicitly state that illegal immigrants do not have voting rights and no verified cases of illegal voting by them in New York exist.
politifact.compolitifact.comDo ‘illegal immigrants now have the right to vote in New York’? No, that’s False
factcheck.afp.comfactcheck.afp.comMusk falsely claims non-citizens can vote in New York City+1 more
What the Evidence Shows
Multiple reliable sources confirm that New York City's law allowing noncitizens to vote in municipal elections was struck down by the state's highest court. This law would have permitted approximately 800,000 noncitizens, primarily legal residents, to vote if it had taken effect.
However, the court ruled that the New York State Constitution restricts voting rights to citizens only, thereby invalidating the law before it was implemented. None of the sources provide evidence or official findings that illegal immigrants have actually voted illegally in New York elections. Fact-checking organizations and major news outlets clarify that while noncitizen voting was proposed, it never became law or practice, and illegal immigrants do not have the right to vote.
The figure of 800,000 refers to potential eligible legal noncitizen voters under the proposed law, not actual illegal voting by undocumented immigrants.