PhotoDNA: "is an image-identification technology used for detecting child pornography and other illegal content which is reported to the National Center for Missing & Exploited Children (NCMEC) [...] It is used on Microsoft's own services including Bing and OneDrive, as well as by Google's Gmail, Twitter, Facebook, Adobe Systems, Reddit, Discord and the NCMEC, to whom Microsoft donated the technology." [1]

I'm (almost) certain any Google service will tag CSAM for reports and will also hash content it deems illegal for the NCMEC too [2], so I wouldn't be surprised if Google Photos already did something like this.

[1] https://en.wikipedia.org/wiki/PhotoDNA

[2] https://transparencyreport.google.com/child-sexual-abuse-mat...