I used to do a lot of fire litigation. So this one interests me. What does the science actually say?

Curiously, in the tests described here, butter did not auto-ignite at temperatures up to 440C (824F).

https://drum.lib.umd.edu/bitstream/handle/1903/11333/Buda_Ortins_ResearchPaper.pdf;jsessionid=28A5F60261B2771E0905EB005E4B3ACF?sequence=2 https://twitter.com/AITA_reddit/status/1248215700898004992

This source indicates that the coils inside a toaster reach temperatures around 600C (1112F). It's not much of a source, but let's use it for the moment.

https://what-if.xkcd.com/155/ 

But the bread itself isn't exposed to such temperatures. It sees temperatures of about 140-160C (285-320F). https://www.cnet.com/news/appliance-science-the-well-done-physics-chemistry-of-the-toaster/

The coils are at the sides of the bread--not below it. The butter isn't likely to see its auto-ignition temperature. So I think that the OP isn't going to ignite the butter on his bread, so long as the pop-up mechanism works correctly. He'll just get soggy bread. But you >

could see a problem if the pop-up mechanism gets gummed up from butter that has melted and resolidified around the mechanism.

Also, and most important, the OP is just The Asshole anyway. This is performative toasting of bread in an assholy way.