Is it not recommendable to stick with plugins that are no longer supported by the author for production sites?
As for me key questions here are:
- Is plugin outdated? Not using approaches and coding practices of current WP versions.
- When will it become outdated? This can range from months (next WP version changes something relevant) to years (plugin does something simple on using common and basic hooks).
- Can you handle when it will become outdated? Maybe you can maintain it yourself. Maybe you have budget to hire someone for that. Maybe it will ruin your site.