I emailed support and they cancelled it for me, and said:
"We are working on cancelling subscriptions being an automated process the user can do on their own."
Support didn't respond to my question as to why my subscription was not listed in the "Billing" section, but it was presumably for the same reason that there was no "Cancel" button.
According to Crunchbase.com, Repl.it has raised $24.6M. That they didn't have a self-serve cancel functionality (and they had a deceptive "Billing" section that omits actual subscriptions) on their Account page deserves community criticism. Please help spread the word so that they do better.
[EDIT]: Someone, seemingly from repl.it writes:
"Hey there, we upgraded our stripe library version and missed a part in our code the relied on the old behavior. That resulted in our billing info not rendering for about a day. Sorry about that! A deceptive "Billing" section is definitely not our intention."
Someone confirmed that they have indeed fixed this. The purpose was to get them to fix the issue (since support just gave me the run around). They appear to have done so, so this issue is closed for me.