By default, Matomo should not use any cookies ideally. If someone preferred more accurate data (eg unique visitors), they should be enabling cookies.
To not break BC, there could be a system setting in Matomo which is enabled by default for existing installations and disabled for new installations. Depending on that the tracking code be changed. Since the
piwik/matomo.js might not be writable for all installations, there's a chance we wouldn't be able to guarantee BC since we do need to modify the tracking code to achieve this. So it would need quite some communication upfront possibly. Mostly, the file should be writable though. Also the generated tracking code would be adjusted depending on this setting so if someone was using WP-Matomo Integration on WordPress where they embed the tracking code into the site through Matomo API, then even if the JS tracker cannot be changed on the filesystem, the tracking code configuration would still enable/disable cookies depending on the setting further reducing the number of people impacted by this.
How do I get rid of cookies in matomo?