There's never been a set definition for a developed country, but you know it when you see it, and you know a developing country when you see it.
A very basic criteria is where you cannot see a single child labourer or beggar, and where all children are housed, fed and given adequate education regardless of what happened to their parents. All the homeless in the west are adults, you won't see children begging on the roads like in India and most of the developing world. And high school teenagers doing part time work at McDonald's doesn't count as child labour.
A developed country is also where people in trade professions like plumbers, electricians, construction workers and truck drivers enjoy a middle class to upper middle class lifestyle similar to engineers.
Developed countries today have serious problems of their own, and the US and South Korea are two of the worst examples. Better examples are Scandinavian countries, France, Germany before 2022, Austria, Netherlands etc. Of course there's a scale difference, but the EU overall is 500 million people which is no small number.
I would consider a country with even Poland's and Czech Republic's current living standards as a developed country.
But to get a true picture of what I mean by developed country, one will have to look at 1950s America where even a car mechanic could afford a suburban home, support a family of 5 with one income, and not pay too much for healthcare and education for his kids. Of course, segregation was a major problem then, but you get the picture. The US dropped the ball during the 1970s.