>>48330535+ boosted raids + Injected raids + anyone who ever forced a shiny via powersavers + anyone who owns Pkhex + event shinys/fateful encounters.
In fact of the 1000s people ive played long side i doubt 50 people even had a none Square shiny Star is by far rarer if you consider shinys rare.
>>48330555using as a example older Pokemons square means you hit the 1:63K thing with the SID being matched so older Pokemon would be 99% star