I was never really sure what the "Smaller File (8-Bit)" option did in the Export As window until I read up on the PNG format. For the longest time I was confused becuase I assumed the "8-Bit" there meant 8 bits/channel, which is usually what people mean when they say that. And also many people see the 8 bits/channel mode in the Image > Mode menu. I would never have guessed that it meant 8-bits worth of color indices in a palette, therefore only 256 colors total.
Even after reading how PNG has a few modes such as indexed pallete vs TrueColor, the "8-Bit" wording is still confusing. And the "smaller file" wording implies a kind of compression, and doesn't nearly convey the magnitude of the change, going from 16 million colors to just 256. The current wording also doesn't make it obvious why semi-transparent pixels are changed to opaque.
Changing it to say "256 Colors" should solve all of these and make it immediately obvious what the option does.
And I came across this thread which shows a lot of people have the same confusion: https://community.adobe.com/t5/photoshop-ecosystem-bugs/p-export-as-8-bit-png-issue/idc-p/13883218/p...