To give you an idea of just how long some of these combo AUs have existed for; the original OuterSwap post is from December 29th, 2015, making it one of the oldest combo AUs of all time.
www.tumblr.com/telokinetic-...

What I'm about to show you might be one of the funniest things I've ever seen. No joke; this is what canon OuterFell looks like.
drawception.com/panel/drawin...

The oldest DustFell post was actually referred to as "MurderFell" (i.e. Murder Sans from Dusttale + Fell) and posted to Tumblr by ilovygsyun on May 23rd, 2016:
www.tumblr.com/ilovygsyun/1...

Fun Fact: This one post was responsible for the creations of THREE different big AU + Shift AUs.
gall.dcinside.com/board/view/?...

The identity of the person who made this ShiftFell artwork is one of the biggest mysteries in Canon Archive history. We know it was from January 2016, and likely the oldest ShiftFell, but we have no idea where it was posted or who made it.

If you're curious what canon SwapDust is like, it's on DCInside as well! It was posted on March 26th 2016 by someone named "박는다", just 8 days after DustSwap. It's not really anything to write home about, though - just a single sentence of lore.
gall.dcinside.com/board/view/?...

Butterfly Effect actually isn't the canon version of DustSwap. It was considered canon for years, but an older post was found just a couple months ago.

This is the real canon DustSwap, posted on March 18th 2016 to DCInside. gall.dcinside.com/board/view/?...

The source of our profile pic is actually the canon version of FellSwap. It was created on October 25th 2015 by Tumblr user woahlookartandwords, just 5 days after Underswap! It was also, to our knowledge, the first combo AU ever made.
www.tumblr.com/woahlookarta...

Welcome to the Canon Archive Bluesky account, where we'll be sharing fun facts about the canon versions of Undertale AUs! Check the thread below if you want more information on what this is actually about.