Joomla! 3.10 is slowly moving towards End of Support (EOS). There is less than half a year left for bug fixes. After that, one year for security fixes until 17th August 2023. Joomla! 4.x, the new major version of Joomla has already been available for almost a year and offers many improvements. In addition, you benefit from the latest production versions of PHP which bring security and performance boosts to your site too.
So although you could still start new projects with Joomla! 3.10 we highly recommend to use Joomla! 4 for new projects.