They can be synced&backed-up just like it's suggested here: https://bugs.telegram.org/c/8307 .
There's also a similar suggestion ( https://bugs.telegram.org/c/904 ) but it only reports the lack of secret chats on Telegram Desktop*, when Telegram Web-browser also lacks them.
(Obviously secret group messages don't actually exist but I hope that they will be implemented soon**, along with extended chats: https://t.me/durovschat/518625 )
*secret chats are already present on macOS
**this function is also suggested here: https://bugs.telegram.org/c/1231