I had the exact same issue as Arlo. Running 10.14.6 and updated Safari to v13 yesterday morning and immediately regretted doing so.
It appears that the ~/Library/Preferences/com.apple.Safari.Extensions.plist file existed prior to Mojave. I was able to recover this file with TimeMachine but the most current file was from early January 2019... about the time I upgraded to Mojave... this plist file did not exist in any TimeMachine back up since then. Using the SRRecover tool I was able to restore my SessionRestore sessions to v1.5.2 (purchased this morning) as of that January 2019 date... and for the record this worked flawlessly... THANKS RUSSELL!
Here's the question... where did SessionRestore store session data after a system was upgraded to Mojave?... in other words, are my stored sessions from early January 2019 to now also recoverable?
Thanks for your time.