Sur Debian je ne crois pas que ça soit conseillé effectivement... mais
pas plus que pour Ubuntu !
Cf la doc en anglais : "Ubuntu supports the ability to upgrade from
one LTS to the next LTS in sequential order. For example, a user on
Ubuntu 16.04 LTS can upgrade to Ubuntu 18.04 LTS, but cannot jump
directly to Ubuntu 20.04 LTS. To do this, the user would need to
upgrade twice: once to Ubuntu 18.04 LTS, and then upgrade again to
Ubuntu 20.04 LTS."
Source: https://ubuntu.com/server/docs/upgrade-introduction
Après, oui, ça peut fonctionner, chez Debian aussi.