In short: make sure that the Jetpack plugin is installed and enabled, and also that the Jetpack Lite plugin is not enabled.
I found out that Edit CSS is provided not by WordPress itself but by the plugin Jetpack. Jetpack was still installed; I had a Jetpack menu right below Dashboard. But when I opened it, I saw only two items, “WordPress.com Stats” and “WP.me Shortlinks”, neither of which have to do with CSS.
When I went to Plugins > Installed Plugins in the dashboard, I saw that the “Jetpack by WordPress.com” plugin was enabled. But right below it was “Jetpack Lite”, which turned out to be the culprit.
The purpose of Jetpack Lite is to “[Disable] all Jetpack modules except for Stats and WP.me Shortlinks modules”, for the sake of simplicity and saving memory. This means that it disables all other features of Jetpack, including the Edit CSS functionality. After I clicked Deactivate under Jetpack Lite in plugins, the Edit CSS menu reappeared and the site styles returned.
See also WordPress › Support » How to restore Edit CSS if Jetpack plugin was deleted?. It explains that CSS customizations in Edit CSS are saved separately from the plugin, in the wp_options table of the database, and that re-enabling the plugin will load the already-saved styles.