
This user has been denied support.
admincp - admin/index.php?module=tools-system_health&action=utf8_conversion
Make a backup in any case before performing database conversions.
Google SEO recommended 'utf8' charset before which is outdated. 4-byte UTF-8 characters used to be exotic (some chinese characters and other unusual scripts) but became more prevalent with emoji and stuff.
Also a few years ago it was still common to find outdated MySQL server that didn't even support 4-byte UTF-8. Nowadays it should not be a problem anymore, so the default choice should be utf8mb4, not utf8.
Even if you are not planning on using such characters actively, weird behavior occurs when users post them anyway. Switching to utf8mb4 should resolve that issue.
Even so, if you didn't have issues until now, it's also fine to leave as is. You can ignore this message and treat it as informative.