Thanks for the replies guys.
My main focus was whether or not Sync can be used as a backup, and it seems it cannot. Sync, I guess, doesn't store sessions you can recall nor go back to. Good to know, I guess. The best course of action here is to save a session (File -> Save Open Tabs as Session) just in case anything goes bad.
But for those wondering, yes, the extension was "The Great Suspender". The option to reactivate the extension is actually disabled, and even if it wasn't, it wouldn't restore the tabs. The workaround is to check in your history for this garbled mess of characters: "klbibkeccnjlkjkiokjodocebajanakg" and copy/paste them, sans the extension prefix.
I honestly hope Vivaldi includes a timed interval for its hibernate feature, as well as a whitelist for sites you never want to invoke hibernation on (unless manually). I'd rather not have to use an extension if I can avoid it. From what I understand, Microsoft Edge has this exact feature... But, for the time being, I'm fine with just manually hibernating things and utilizing lazy-loading.
Never thought I'd see the day when MICROSOFT'S browser has a useful feature that neither Google nor Vivaldi has... π π