I am literally begging the world to toss the term "third world country" into the trash, take out the trash, incinerate the trash, and forget the trash ever existed. I actually studied political science in undergrad and over a decade ago, we were discouraged from using that term. Because 1) it is rooted in Cold War divisions (first world-US sphere of influence, second world-Soviet sphere, third world- everyone else) and we are no longer in the Cold War. And 2) it carries a lot of racist, colonialist undertones.
It shocks me that "reputable" publications here in the US still use this term. It is useless, it is inaccurate, it is racist. Please stop.
"Underdeveloped" is just as bad. Maybe stop trying to find a polite term for countries that got fucked by colonialism and just admit that a bunch of the world got fucked by colonialism.