Not only that, but half of the '50's shorts and most of '60's shorts seemed to be removed for unknown reasons.

Strange. I wonder if the Gumby partnership with KaBillion is coming to a close soon?

I don't know. The videos were gone before they recently uploaded the six '80's shorts.

While the 1950's and 1960's Gumby shorts have been disappearing from Kabillion's YouTube page, it looks like another online streaming service has them available, Ameba TV. Right from their homepage, they have links to 1950's, 1960's and 1980's episodes. The 1950's looks to be complete. The 1960's looks to be mostly complete but there are some missing (notably those with plots involving Native American characters). There appears to be approximately 60 1980's shorts, which last I checked was more than what Kabillion had on YouTube.

http://www.amebatv.com 

I believe a number of the 50s episodes are still on YouTube, but now they're unlisted. I found some in my favorites. (And I shared "Train Trouble" on a model train group on Facebook.)