WordPress root .gitignore. Ignore everything and then opt-in (exclude from the ignore) for your custom code.

.gitignore

# This is a template .gitignore file for git-managed WordPress projects.
#
# Fact: you don't want WordPress core files, or your server-specific
# configuration files etc., in your project's repository. You just don't.
#
# Solution: stick this file up your repository root (which it assumes is
# also the WordPress root directory) and add exceptions for any plugins,
# themes, and other directories that should be under version control.
#
# See the comments below for more info on how to add exceptions for your
# content. Or see git's documentation for more info on .gitignore files:
# http://kernel.org/pub/software/scm/git/docs/gitignore.html

# Ignore everything in the root except the "wp-content" directory.
/*
!.gitignore
!README.md
!.scrutinizer.yml
!apigen.neon
!wp-content/

# Ignore everything in the "wp-content" directory, except the "plugins", "mu-plugins",
# and "themes" directories.
wp-content/*
!wp-content/mu-plugins/
!wp-content/plugins/
!wp-content/themes/

# Ignore everything in the "mu-plugins" directory, except the mu-plugins you
# specify (see the commented-out examples for hints on how to do this.)
# !wp-content/mu-plugins/my-plugin.php
wp-content/mu-plugins/*

# Ignore everything in the "plugins" directory, except the plugins you
# specify (see the commented-out examples for hints on how to do this.)
# !wp-content/plugins/my-plugin.php
wp-content/plugins/*

# Ignore everything in the "themes" directory, except the themes you
# specify (see the commented-out example for a hint on how to do this.)
# !wp-content/themes/my-theme.php
wp-content/themes/*

I didn't come up with this. I am not sure the source, possibly another gist like this: https://gist.github.com/redoPop/444295