How to make Git “forget” a file that was tracked now it is in .gitignore?

For that, you should update the git index, and git update-index does that job.

You should use --skip-worktree which is for modified tracked files that the user doesn't want to commit anymore and keep --assume-unchanged for performance to prevent git to check the status of big tracked files.

git update-index --skip-worktree <file>

# or
  
git update-index --assume-unchanged <file>