First issue for secret chats on Telegram desktop github repo dates back to Jun 1, 2014. Despite being requested many, many times since then, Telegram development team refuses to work on adding secret chats to it's Telegram desktop client, Claiming It's hard and a lot of work and they won't even merge a PR that implements this feature.
In Aug 23, 2021, a crowdfund was started to implement this feature in three phases:
a community member named sergiotarxz, who had no prior experience with Telegram desktop code base, started working on the issue on May 20, 2022 and finished phase one on May 29, 2022, in only 9 Days, and phase two on Jun 17, 2022 in around 50 days.
All the functionality related to DH key exchange is also present in Telegram Desktop, because mtproto use it for transport layer security and cloud chats.
And of course there is absolutely no need to migrate Telegram Desktop to TDLib in order to implement secret chats.
Here are some of the misleading arguments on their FAQ about
Why are there no Secret Chats on desktop apps?
Desktops are a much less personal environment, frequently monitored at work and left unattended at home. It’s easier to keep an eye on mobile devices.
FALSE. They are not less personal; laptops and towers can be even more personal than mobile devices. On desktop apps we log in with the same frequency as on mobile due to same session-preserving mechanisms. Most people don't let network administrators or family members to access their personal desktops, but in case they do, they should be aware of security risks and take measures to avoid personal data leak (why not writing about this instead of making misleading statements?). We often leave our mobile unattended and many people don't even use proper security locking mechanisms on Android phones; desktop users are covered by automatic screen locking and login screen; many desktop users also use full disk encryption. It's also easy to keep an eye on your desktop if it's a laptop, and tower desktops can be protected by full disk encryption. Additionally, the probability of having a mobile device getting stolen or lost is significantly higher than for desktop.
Open systems vs. Sandboxed systems ...
FALSE. There are lots of efficient sandboxing solutions for desktop, such as firejail or using VMs/containers, which are very secure. To have Secret Chats, you need your app to store data on the device, and implementing this is no way more complicated on desktops, and especially the web. how else browsers keep users safe? if their storage is not safe then all users on the web are in danger of getting their credentials stolen. The Web and Desktop apps currently get most data from the local storage on startup and don't discard it when you quit or log out, so there is no point to mislead users stating the otherwise; cloud clients synchronise the data with the local storage in most use cases. This means you shouldn't lose your secret chats at all because they should be stored on the local storage, in the same way as other desktop applications do.
A chaos of chats with the same person If each chat only connects two devices, users get many different chats with the same person. Imagine two people that have a laptop, a desktop at work and a mobile device each. That’s 9 secret chats in total, and most of them are useless at any given moment.
This is a possibility, not a common problem. If someone absolutely has to encrypt all their messages on all of their chats, they should be able to. People who use E2EE chats are mostly aware of how they work.
And if that wasn’t enough, desktop sessions tend to be a lot more transient. One of the important features of secret chats is that they are destroyed on your device when you log out. Most desktop users log out rather frequently. Telegram Web users do this even more often.
Desktop sessions work in the same way as mobile sessions. Secret chats are destroyed on your device when you log out and this is why they are secret, nothing wrong about it. Most desktop users don't log out frequently and there is a big lack of context in the original statement. Telegram Web users should know better why they use Web client, it's their own choice, however, all this is not a valid argument to not implement secret chats on Web client either, since web-browsers are even used to store cryptocurrency (Metamask and such), and there is no problem with that for people who are aware of security risks; there is no problem enlightning people on security risks associated with web-browsers usage, this is what Telegram team should do instead of writing lots of misleading reasons to not implement Secure Chats.
Since Telegram Desktop is an open source software, we, the community, can implement the feature without needing to wait for official developers to do so. Hence we have created an initiative to speed up the implementation and will sponsor the devs who will work on it. This work currently is happening in this GitHub issue [Feature Request] Secret Chats in Telegram Desktop #2 and you can contribute to it right now.
Github issues that have been opened regarding secret chats: #5 #68 #73 #118 #162 #236 #246 #363 #598 #619 #654 #777 #871 #1545 #1958 #2610 #3731 #4416 #4438 #4517 #4518 #4591 #5080 #5493 #6491 #6701 #6770 #6771 #6953 #7563 #7934 #8050 #8071 #8091 #10162 #16388 #16772 #16835 #16878 #16938 #17216 #24510 #24829