Make sure you export your old data out first. This won't show your old sessions. Plus your sessions will disappear if you reset safari or clear history apparently.
I am aware of this workaround, but do not know how feasible it is across browser restarts and longer term. My understanding is that any extension so loaded is unloaded every time Safari quits.
https://sixcolors.com/post/2018/09/give-new-life-to-old-extensions-in-safari-12/
Make sure you export your old data out first. This won't show your old sessions. Plus your sessions will disappear if you reset safari or clear history apparently.