This warning shows up when using PHP 7.3+.
If you are not using webservices (you probably are not) then you can disable them in the Geeklog Configuration which will prevent the errors.
I'll add a comment to our PHP 7.4 compatible improvement on Geeklog about this problem.
https://github.com/Geeklog-Core/geeklog/issues/1079
Thanks
One of the Geeklog Core Developers.