Darktable does not appear to offer a true “lock” for an image’s history stack in darkroom. A practical workaround is to duplicate the image in lighttable once you’re happy with the edit. In darktable, duplicating creates another sidecar/XMP version, so your finished version remains preserved while you continue experimenting on the duplicate. If grouping is enabled, you may only see the most recent version until you ungroup the images. This is the safest approach mentioned for protecting a completed edit from accidental changes.