A lot of people overblowing this in the replies right now. Machine-id is mainly used by fleet admins at companys to manage individual devices, to generate system application keys, or tag logs (and yes systemD provides this file, but so does dbus and many non systemD applications depend on it). Chrome and Firefox might access your machine-id but their also tracking you in much worse ways by default aswell, if you care about privacy your probably already using an alternative browser that dosent use your machine-id

The problem isnt windows or linux having a machine-id but that your applications are not properally sandboxed (flatpak does not sandbox /etc by default, so no, flatpak is not a solution) or that your using applications that handle security poorly