If the project is not usable in its current form, say something to that extent near the beginning of the README. It doesn't have to be a bold big all-caps "Do not use", but it should be noticeable to an idiot in a hurry to save everyone's time.
If the project is usable but in a dire state, you should probably say in rather clear terms that you are not maintaining the project and (if you want to be nice) suggest alternatives for people who are using the project or would want to use the project. If the project is registered on package manager repositories like NPM, RubyGems, PyPI, etc, it's probably also good form to publish "one last update" to update the description -- but don't have your package emit additional console output unless the user can disable it.
If the project is usable but you need to take a hiatus, make this clear in any open issues. It's good to know that a project isn't actively maintained at the moment but not dead, because otherwise you may not think to have another look when the maintainer resumes their work (or a new maintainer has taken over). Also do this if you are actually looking for maintainers: if someone went through the trouble of creating a PR or raising an issue, they may be interested in contributing further or at least tell others.
Additionally, if you have a lot of active and a lot of inactive repos cluttering up your account and you find the inactive ones too distracting, you could create a GitHub organization and move the unfinished/exploratory projects there. It won't be an obvious indicator to others, but you may find it soothing. I may have gone overboard with this (I also moved all my one-off forks into a separate org), but I think it's more humane than just deleting abandoned projects.