So, I accidentally wiped out all my extensions / groups by uploading a CSV without reading the fine print, and unfortunately don't have a backup. Is there any going back?
No, not really.
Splunk has no built in versioning, nor does it even "make a backup" before overwriting things.
You should set up backups of your Splunk environment. Of greatest importance is capturing your $SPLUNKHOME/etc folder, which includes your configuration information, dashboards, searches, alerts, lookups and things like that. This is as simple as setting up a cron job to, each night, tar up the contents of those directories. If you then save that file onto another system you at least would have recourse to "Ooops I overwrote a lookup I needed".
If you use any number of mechanisms to save a running week's worth of those (renaming them, logrotate, whatever), then suddenly you have at least the core set of information to rebuild your stuff if it goes away.
If you need to back up the data, well, this gets trickier but the docs have you covered.
Anyway, even a simple backup is better than nothing, so I'd suggest setting at least something up. But I suspect you now have. 🙂