IMO the way Darth Vader's suit has been treated in modern Star Wars works, and especially in fandom, exposes a pretty significant generational difference in expectations of medical care between modern writers and people from George Lucas' generation.
Lucas described Vader as being in "a walking iron lung". That simile is no accident.
Before the development of the polio vaccine in the 1950s, iron lungs - monstrous tanks enclosing the whole body below head level, which left their users lying flat on their backs and mostly immobilized - were the only means of keeping people alive if polio paralyzed their lungs. Polio was absolutely terrifying, and Lucas grew up in the 1940s and 50s, so he'd have known how feared it was. But until Jonas Salk developed the vaccine, there was no alternative to the iron lung, aside from never catching polio in the first place.
Likewise, although there was medical treatment for burns including the use of skin grafts, extensive burns were very often a death sentence. But since the 1960s, burn treatment has come a long way, allowing patients to survive injuries that at the time of World War II would have been fatal. Silver sulfadiazine was developed in the 1960s as a topical cream used to prevent infection, which revolutionized the survivability of burn injuries. According to this article, in the 1950s, being burned over 50% of the body meant at least a 50% chance of death; by the 1990s, the equivalent burn level for the same odds was 80%.
Vader's suit was conceived with these older standards of medicine in mind: he survived full-body immolation in a way that should by all rights have killed him, and would have, had he not been put in a life-support suit. There was no alternative. His options were the suit or death.
But these days, medical science has advanced so much that such a drastic intervention being Vader's only option for survival seems unrealistic. Few people remember the days before the polio vaccine or silver sulfadiazine. Iron lungs are no longer even manufactured. So the idea that the best medicine the GFFA can come up with isn't up to 21st century medical levels seems alien to a lot of younger writers. (Especially since the GFFA also has things like seamless prosthetic limbs.)
As a result, there's a perceived need for there to be some explanation for why Vader's suit isn't up to modern standards of care. Which is why we now get the concept of Palpatine deliberately making Vader's suit a torture device, designed to leave him permanently in pain in order to fuel his anger and rage, as well as keeping him from challenging his master. Fandom in particular has taken this idea and run with it, but it also crept into things like the novelization of ROTS, as well as encyclopedia-format "official" SW works coming after that film.
I might also note that Vader being immolated was part of his backstory well before Vader and Anakin were combined into one character. In Lucas' interview in the August 25, 1977, issue of Rolling Stone, he mentions the lava duel but describes it as involving three characters, Vader, Luke's father, and Kenobi. Nonetheless, according to the 1997 Annotated Screenplays book, this didn't stop him from momentarily considering an alternative idea in his second draft of ESB from April 1978 (the same one that more lastingly combined Vader and Anakin), where he mooted the idea that Obi-Wan instead pushed Vader into a nuclear reactor.