Uninstalling

Deactivating leaves everything alone, so switching the plugin off to test a theme loses nothing. It does ask your page cache to empty, so cached posts lose the list too (see Page caches). Network-deactivating it takes a few seconds, however many sites the network has. LiteSpeed Cache and WP Super Cache are asked once, for the whole network, and NimbleCache, switched on for the network, empties every site by itself. The other caches are asked one site at a time for about three seconds. On a large network that does not reach every site, and the sites it misses keep their cached posts, list included, until they expire: empty those caches yourself if that matters. A site that has also switched the plugin on for itself keeps running it. Network-activate it again and each site empties its cached posts on its first request that is not answered from the cache.

Deleting removes data only if you ticked Delete all data on uninstall under Settings. It is off by default. On a network the tick is per site, and only the sites that asked are cleared. That includes the Table of Contents (sidebar) widget and its place in your widget areas. It also includes a Contents (sidebar) or Table of Contents block you added under Appearance → Widgets, when that block is all the widget holds. A widget where you put other blocks beside it is kept, for you to edit.

While the plugin is switched off, or after deleting it without the tick, a block you added under Appearance → Widgets stays in place and shows nothing. A theme that draws a box round each widget shows that box empty. Remove the block under Appearance → Widgets if you are not turning the plugin back on.