Migrate the D7 variable, search_active_modules.
Most of the variable to config migrations are straightforward. This one is a little different and will require more work. Search_active_modules is a serialized array and the destination is based on the key/value pairs.
For example,
a:2:{s:4:"node";s:4:"node";s:4:"user";i:0;}
is converted to:
search.page.user_search.status=FALSE
search.page.node_search.status=TRUE