Debian 11's ifupdown 0.8.36 does not contain the /etc/network/if-up.d/resolved script that contains the DOMAINS=DOMAINS line.
The equivalent functionality in Debian comes from /etc/network/if-up.d/000resolvconf from the resolvconf package.
(I don't know if these are standard or if these are vendor customizations to the default package lists).
Debian 11's ifupdown 0.8.36 does not contain the /etc/network/ if-up.d/ resolved script that contains the DOMAINS=DOMAINS line.
The equivalent functionality in Debian comes from /etc/network/ if-up.d/ 000resolvconf from the resolvconf package.
(I don't know if these are standard or if these are vendor customizations to the default package lists).