BTW, I looked at the source and it does look like a jQuery UI picker. So, you...

BTW, I looked at the source and it does look like a jQuery UI picker. So, you're right that it's deprecated in Drupal 9. Still, this is Drupal 8.9 core as of right now.

Edit: It could be that bootstrap is enabling jQuery UI on that field. When I took the screenshot, though, I clearly remember that I enabled Olivero and uninstalled Bootstrap.