When one talks about the missing episodes of Doctor Who, they usually have to be separated into three distinct categories: the first is the "partly missing", and those are the serials with one or two missing episodes, and still the ones that are, more or less, 75% there. This is the fate that befell such serials as "The Crusade," "The Web of Fear", "The Invasion", and "The Ice Warriors". However, with that number it usually benefits the stories with six or more parts and can usually be helped along with the animated reconstructions and released as standalone DVD's (though in the case of "The Crusade", with half of its episodes remaining, this has yet to happen, I imagine due to most of the historical serials not being as overtly popular as the sci-fi/horror based ones). The second category is the "mostly missing", and this is the one that proves the most frustrating for completionism. "The Daleks Masterplan" is one such notable case, with only three of this twelve episode epic still remaining. The rather excellent, "The Evil of the Daleks" is another, with only one of its seven episodes still existing in the archives. "Galaxy 4", "The Moonbase", and "The Abominable Snowmen" are other such serials with only a couple orphaned episodes remaining. These gaps are problematic whenever one wants to sit down and marathon Doctor Who from the beginning: while it wasn't quite as apparent at the start, the further one goes into the 1960s era, the more these gaps can get quite annoying, and as such that leaves the viewers scratching their heads in regards to the fates of companions such as Vicki, Steven, Katarina, Ben, Polly, and Victoria (each of whom have stories either half-complete or no longer existing in the archives) or even the fate of the Doctor himself.
So when Twitch started its Doctor Who stream, there were a couple of noticeable omissions from the lineup, and they are mostly from the 1960s. To be fair, those omissions aren't as glaring when one realizes that their existence is, at best, tenuous. They certainly couldn't stream a story like "The Power of the Daleks", which is wholly reliant on an animated reconstruction, for fans to get a flavor of the serial? Twitch must have realized this, as two stories that were shown from Season 5 "The Web of Fear" and "The Ice Warriors", appear to rely on BBC official recons using telesnaps and the like to fill in the missing episodes. However, one glaring one that can't quite be explained is the complete and total absence of Season 4's "The Tenth Planet", notably the first story to feature the Cybermen, the first regeneration, but above all should be remembered for the simple fact that it was the final regular story to feature William Hartnell as the First Doctor. Episode 4 concludes a nearly three year run for the actor, who had seen the show become a household name and established many of the ticks and tricks that would be seen in his successors, each of whom used bits and pieces from Hartnell's portrayal. His run was most notable for seeing the introduction of the Daleks, but along the way, as the Twitch stream revealed, he had his own arc from the distrustful, gruff old man to the kindly grandfather-type that regretted seeing such companions as Susan, Vicki, Ian, and Barbara leave him. But the behind the scenes on it, of a Hartnell so ill that he does not even appear in Episode 3, and a Hartnell who had grown so difficult that John Wiles and his successor Innes Lloyd considered replacing Hartnell at various points in 1965 and early 1966, before the idea of regeneration and making the Doctor a whole different actor with a different personality was even mooted.
I like to think it wasn't a choice made without some heart behind it. As difficult as William Hartnell had likely gotten by that point (not to mention the stories coming out from surviving cast and crew about him) replacing him was not the easiest route to take. Despite a downturn in viewers during Season 3, Hartnell's Doctor was a huge seller to the overseas market, which is why a higher number of Hartnell stories survive than Troughton's. Removing him was a huge risk, and one not taken lightly even if he was becoming a problem. As well, Hartnell himself likely wasn't ready to go. As other actors have opined, it can be quite easy to trick yourself into staying in the part forever. And as much of a stink Hartnell put on about being cast in the beginning, over time he'd grown to love it and become protective of it (other more positive stories center around him knowing what each button on the TARDIS console actually did). But with time itself, and his declining health, it was become all the more apparent that forever was not an option for Hartnell. He had his good days when working proved enjoyable (listen to "The Smugglers", a nice romp with Hartnell in top form) and then he had his bad ones (visually he seems much frailer in "The Gunfighters", as Steven takes on a larger role in the story than previous) and even if he hadn't been forced to leave as he was, it's very likely he may have left on his accord when he couldn't keep up with the punishing filming schedule the show endured in the 1960s.
As such, "The Tenth Planet" was cut from the Twitch lineup for no real reason, other than it likely was they didn't have the clearance to use the animated recon that was done for the missing Episode 4 ("The Web of Fear" used telesnap recons, likely because BBC already had it on hand, whereas "The Ice Warriors" used the previous animated recon from the DVD release, further muddying the reason why "The Tenth Planet" was cut) but even that makes no sense. One could make a case that it was Twitch didn't want to have too many animated reconstructions on their lineup, and I can agree with that, but when comparing "The Tenth Planet" to "The Ice Warriors", my vote will go towards "The Tenth Planet" and its historical importance each and every time, particularly since "The Seeds of Death" is far and a way a more superior Ice Warriors story than their debut, and I would never say "The Web of Fear" should not be seen, because it is a fantastic story that I'm glad to have recovered.
So what's all that exposition got to do with the picture above? Episode 4 is one of the BBC's most sought after Doctor Who episodes, mostly because it is Hartnell's final appearance. It's great to have the final few moments showing the regeneration, but this lack of foresight on the BBC's part robbed fans some fifty-two years later of watching Hartnell bow out gracefully on Twitch, with his final appearance during the live stream being "The War Machines", a decent enough story with an okay Hartnell but not the farewell he needed to say to the new fans who'd fallen in love with him in the same way audiences did from November 23, 1963 until October 29, 1966.