Which background task looks for pg_xlog in 10?

I see this in /var/log/postgresql/postgresql-10-main.log:

postgres postgres@template1 ERROR:  could not open directory
"pg_xlog": No such file or directory

postgres postgres@template1 STATEMENT:  SELECT count(*) AS segments
FROM pg_ls_dir('pg_xlog') t(fn) \
WHERE fn ~ '^[0-9A-Z]{24}$'

Where does this come from?

Regards
Johann
--
Because experiencing your loyal love is better than life itself,
my lips will praise you.  (Psalm 63:3)