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!
I know, right?
I’ve been using it since forever and I can’t imagine living without it! 
But I have a good news! 
I’ve contacted him and we agreed on some collaboration - migrating existing images from his servers to mine (millions of images!).
But I won’t be taking over his addon, I’ll make a new one.
There will be an official statement soon from me and him…
I’ll will also start some crowdfunding campaign to help me finance it.
Wish me luck! , it will be a bumpy ride for sure!
Wow! That’s pretty big news! 
Looking forward to it!