fof/night-mode is not required in your composer.json and has not been removed
./composer.json has been updated
Running composer update fof/night-mode
Loading composer repositories with package information
Package "fof/night-mode" listed for update is not locked.
Nothing to modify in lock file
Installing dependencies from lock file (including require-dev)
Nothing to install, update or remove
Generating optimized autoload files
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but its inside /vendor/fof i see it there
the json still remained with
core "^ 1.0",
and other as "*",