Warning: SPOILERS ahead for Supernatural season 15, episode 18, "Despair".
With only two episodes to go, Supernatural seemingly killed off Castiel for the last time; however, the moment proved controversial for several reasons. For 15 seasons, Supernatural has told the story of Dean Winchester (Jensen Ackles) and his brother Sam (Jared Padalecki). As per the family business, they traversed America, saving people and hunting things. Over the years, they faced everything from wendigo and vampires to demons and rogue angels. Their unconventional family has fluctuated in that time, with their trusted ally, the angel Castiel (Misha Collins), most consistently returning to their side.
That fact has remained true throughout the final season, as Team Free Will has battled their biggest threats ever: Death a.k.a. Billie and God a.k.a. Chuck. In an effort to defeat the former once and for all, and save Dean's life, Castiel surrendered to a deal he'd struck previously with The Shadow of The Empty. With Castiel experiencing a moment of true happiness, the otherworldly entity, as promised, came forth to absorb his soul... taking out Billie in the process. This wasn't the first time Castiel gave his life for the cause. That being said, this time had an air of finality, given the more poignant nature of his farewell. Equally, the moment harkened back to Castiel's first introduction - he left a handprint on Dean's shoulder both times - thus bringing things somewhat full circle.
In the lead up to his sacrifice, Castiel poured his heart out to Dean. The angel told him how he'd always wondered what his moment of true happiness would look like, since what he truly wanted was something he couldn't have. However, he'd come to realize that happiness didn't come from the having of something, but merely from the feeling and the expression. It was then that Castiel tearfully told Dean that he loved him. Fans immediately took to social media, believing this to confirm that Destiel (the fanfic-inspiring ship of Castiel and Dean) was now canon. It would be difficult to argue against such statements. After all, though the group has often expressed love for each other, Castiel's final words had more than a tinge of the romantic - especially, with Dean being overtly heterosexual, in Castiel being unable to have what he wanted. Amid all the celebrations of the confirmation, however, there also brewed some understandable backlash because of Supernatural's history of "queer-baiting" and more.
For much of its run, Supernatural has been accused of "queer-baiting" - teasing a romantic dynamic (mostly for humor) but never committing to true LGBT representation. The concept has been a divisive one among the show's fandom, with just as many disputing the validity of Destiel as backing it. Castiel's latest declaration has only fueled such debate. After all, Dean never returned the sentiment, leaving it entirely one-sided, and will potentially never have to actually acknowledge or address it. Plus, Castiel's statement has already been widely viewed as too little, too late. It was great to see the long-serving character finally express his true feelings (and sexuality), as well as Collins truly nail the performance. However, many deemed it as retroactively overriding the previous queer-baiting examples by making Castiel canonically LGBT - not to mention silence the aforementioned accusations as it finalizes its legacy.
A large portion of the criticism has come from the fact Supernatural went from said queer-baiting to the egregious bury-your-gay trope in a single bound. Several shows have been guilty of this entertainment faux-pas in recent years. In fact, many CW offerings have been particularly called out for it. None have ever accomplished it in such a rapid fashion, however. Castiel barely finished uttering the words before he was dead. Of course, with two episodes still remaining, there's still a chance Castiel could make yet another miraculous return. As such, it would be interesting to see how his declaration will be dealt with. As it stands, however, it looked very much like Supernatural attempted to have its cake and eat it too.