It must have been introduced with Android 2.1 aka Eclair.
Though I cannot name a date (as I found no source for that), we can get a raw estimate. But to advance that, let's first take a look into history:
The first Android smartphone sold was the HTC Dream (also known as T-Mobile G1). As you can see in the device specs when following the link, this device had a little internal-storage (256 MB; also known as "device storage"), and featured a microSD slot. So no "internal SD card" here at all.
A bit later, we saw the first Nexus1 device released to the wild: The Google Nexus One came – you won't believe it – with a microSD slot and, no "internal SD card". Release date here was January 2010, and it shipped with Android 2.1 (aka "Eclair"). Mark the date and the Android version.
Now I've found an article on Android Browser Forensics dated 9/2010 (somewhere I've stumbled upon a copy dated 6/2010, but cannot find the link currently). They report having used the Samsung Galaxy S i9000, which according to GSMArena was released 6/2010 with Android 2.1. Quoting (emphasis mine):
Currently I’m running Android 2.1 Eclair
Each option is checked by default. Click Capture. A folder is created on the internal sdcard called ‘forensics’ with 6
As you can see: This device was running Eclair and featured an internal sdcard (unless the blogger made a mistake here). As usually the Nexus devices are "state-of-the-art", but the one originally shipped with 2.1 had no internal sdcard (which is quite untypical for a Nexus – but this was the only one) – and another device introduced about 6 month later, also running Eclair, features such, I think it's pretty safe to assume that was about the time eMMC was introduced to Android devices as "internal sdcard".
Not a proof, but at least a "good shot" I'd say.
Now we come to the quote of your question: allowing internal and external storage to each be on the same partition was the next step, and introduced with Honeycomb (aka Android 3.0), with the switch being made from USB mass storage to MTP. This brought two advantages to the user: for one, being on the same partition there was less "wasted space". You no longer should run into the condition to get an insufficient-memory error with plenty of free space on your internal sdcard. And number two: this storage stayed available on the device even if mounted to a computer (so no apps crashing and widgets disappearing for that cause anymore). But it also has a disadvantage: most external data-recovery tools no longer work on the connected device, as they need "direct block-level access" – which MTP cannot provide.
So much for a short story on the "internal sdcard" – hope I not only entertained you a little, but also was able to answer your question, at least to a degree :)
1: For a history of Nexus devices, see this Wikipedia article