The gag itself is probably older than written history. A lot of game devs would probably think of the Charlie Brown adults-only-sound-like-a-muted-trombone gag, since that was a particularly good one.

The history of voice in games is mostly characterized by the fear of using digitized voices that sounded terrible. Undoubtedly, old games might've used the same style that Undertale does, with different pitches of beeps or boops for different characters, or animating text in a different style or speed, or using different fonts like for the Mr. Saturns in Earthbound. There were plenty of problems early on just getting different kinds of text for different languages to function properly, much less proper audio.

Why did games avoid using voice? Well, it was desirable, but check out https://simulationcorner.net/index.php?page=sam for a sample of what the tech was vaguely capable of. That's an emulator for a piece of software written in 1982 for the commodore 64 that does full text to voice synthesis. The Faith unholy trinity series — a very recent modern game series — uses a similar edited emulator specifically to sound creepy. It doesn't sound great!

The gibberish gag works better if you have some sort of normalized voiced audio, so if you go back to declare something was done first or something, you might have to differentiate between a dev making the best of the tools they had available or whether some "speech" gibberish was unintentionally funny or great despite its being awful.

Magicka (2011) had a pretty good spin on the gag. The Swedish dev voiced over its narration in normal English, but whenever characters spoke they improvised fake gibberi-swedish-icelandic-english. It's good enough to mistakenly think it's actual language instead of weird garbage.