To me myself, I basically don't know a consistent answer to why they're not uploading classic clips on their official YouTube channel anymore, especially for the popular "#ThrowbackThursday" thing. One thing could be it being affected by the notorious Internet leak of Episode 847's footage recorded from the MoMI's Sesame Street Lost and Found event, given that anyone who supported the workshop's copyright holders stated that leaking this footage "would cause serious effects to future Sesame Street events for lost episodes" and that "if all this leaks now, Sesame can withhold the rest of the footage, because they have lost trust in the fanbase", but I could be wrong. It's highly unlikely they'll stop releasing anymore classic content on behalf of this leak, given that a lot of unauthorized uploads of Classic Sesame Street footage (pertaining to clips that have never been on home media and have been off the air for decades) have been appearing on YouTube and other sites for years.
Now, given by some of you folks pointing out that they're trying to focus more on original work, I'm also beginning to draw another path of my mind to thinking that a clearer reason they're not uploading anymore classic clips at this time is because since the global COVID-19 impact is a seriously dangerous situation to human life at this time, they're focusing much more on trying to teach kids, teens and adults alike about social distancing and staying at home to be safe. They think that's what's more important than classic clips at this time, as again, due to the COVID-19 situation being serious. I see their point, and at the same time, I'm thinking they're like "Okay– For now, let's put a halt to uploading classic material for a while so we can focus on making original content teaching our audience important lessons on social distancing of being 6 feet apart, wearing a mask, staying at home, and being safe. The rest of the classic material we've planned to release, including classic episodes with anymore classic clips that come with it, can go to HBO Max and the AAOPB at this time." Now even though I'm just predicting that's what they think to themselves about releasing anymore classic clips, the last sentence in quotes I made up pretty much seems to have a good point explaining why they cannot release anymore classic clips on YouTube at this time.