Are you sure there was a fire at Sesame workshop, because there was a fire at where they do the German Sesame Street. Besides, I’m sure there’s been a lot of stuff recorded from people that we don’t even know about
Mmm. Well, now Heather's comment got me tad confused. Maybe she's collecting evidence from incidents that happened at television studios for other television programs from back in the day, and she may be using the same hypothesis with Sesame Workshop just from seeing the B&W kinescope copies of the episodes. On the other hand, Gavry3 is right, none of the Sesame Street stuff is lost for sure, they're just not available to the public. But yeah, you're correct that there really was a fire at the studio where they produced Sesamstraße.
On the other hand, I also remember what happened to the studio where they did Iftah Ya Simsim (Arabic Sesame Street)-- during the Gulf War, Iraq forces partially destroyed the studio and stole all the master tapes + the Nu'man costume and Cookie Monster. It's been stated that the master tapes for Iftah Ya Simsim along with the Nu'man costume and Cookie Monster were never recovered and remained lost forever. Strangely, I can still see hundreds of thousands of Iftah Ya Simsim episodes surface on YouTube from time to time. Maybe those came from backup copies found elsewhere.
Despite this, there was even a similar doubt with the 1973 anime of Doraemon which I read more about on
the Lost Media Wiki page for it. Unlike Sesame Street however, some of the master tapes for the Doraemon '73 episodes indeed are lost for sure.
Trust me if any Sesame Street stuff was ever lost, they would not be able to release it on home media nor post it on sesamestreet.org nor their official YouTube channel. Well, good thing at least they have their stuff stored in the archives.