Upgrade guide for 8.0 -> 9.0 and 9.2 -> 9.3: https://github.com/laravel/cashier/blob/9.0/UPGRADE.md
Upgrade guide for 9.0 -> 10.0: https://github.com/laravel/cashier/blob/10.0/UPGRADE.md
There are nothing in the docs about it
That's not true: https://laravel.com/docs/5.8/billing#upgrading-cashier