Reporting that as of release 13.1 for SearchPreview add-on, the sole developer/maintainer has confirmed that maintenance by them will stop sometime at the end of the month (October).
See release notes for more info:
Pretty sad to see you go. I’ve been a long time user but you were one of those that [was] at least ported over to Webextensions when the time came and did so fairly quickly even.
Thank you for your contribution to the community and for keeping the dream alive!