I've already had a go at getting it working but don't know PHP, thanks!
Can anybody get this extension working for MediaWiki 1.31.0?
This will be cool. For the time being I have added a note to the docu that this is an unmaintained extension.
Not sure if this is any help but this is as far as I got trying to bring the extension up to date myself https://github.com/JoshBigGuns/ForceLogin