Hi, I get what you mean, it can feel odd when something that used to work just disappears.
The reason you’re not seeing the option now is that your new desktop doesn’t have the Send to Kindle feature installed, which was likely set up on your old computer through either the Kindle app, the Send to Kindle app, or a browser extension.
The easiest fix is to install the Send to Kindle app from Amazon’s site, sign in, and you’ll get back the right-click and drag-and-drop options.
You can also use your Kindle email address to send files directly; Amazon lists it under Manage Your Content and Devices then go to Preferences then click Personal Document Settings.
Quick question: do you remember if you were using the desktop app or a browser extension before?
That’ll help me point you to the exact setup you had.
Regards,
Aron