Edit: The question has been resolved. For those who don't know and want to know, Charles He explained how to edit default user account settings, including formatting options. As this is something I could and should have been able to figure out on my own, this post should be considered nothing more than a self-own.
It's been in beta for what seems like over a year now. It's jarring to have Markdown as the default when EA Forum Docs is so much better. For example, every time I hit "Ctrl+I" to italicize a string of text but I forget to switch from Markdown to EA Forum Docs, a pop-up window with page information opens.
I'm aware Markdown may have some advantage of making it easier to format in some ways. I'm willing to debate any number of other forum users on the value of whether EA Forum Docs versus Markdown is a better default formatting option. If the Online Team at the Centre for Effective Altruism is being pressured to delay the switch to EA Forum Docs as the default option by self-appointed defenders of Markdown, I will come to their defence. Presuming many others would consider the switch to EA Forum Docs as the default option to be sufficiently valuable, and that a lack of capacity is what has been preventing the CEA's Online Team from completing the process, I would be willing to raise funds earmarked to hasten such an outcome.
I'm aware it may seem ridiculous that this matters to me so much. Yet what's even more ridiculous is that EA Forum Docs is still in beta. I hate Markdown so much.
You probably have set your user to use Markdown, specifically. Go to your user settings, open "site customizations", and check that you don't have "use markdown" set.
Thank you for providing this information as a public service. This comment will be the last one I ever post with markdown as the default setting for formatting.