Like most other distributions, the Debian project decided to end the
separation between the root and /usr filesystems years ago.
Unlike most others, though, Debian is still working on the implementation
of this decision. The upcoming Debian 12 (“bookworm”) release will
feature a merged /usr in most respects, but there are a couple of
nagging issues that threaten to stretch this transition out for some time
yet.

Source: LWN.net – [$] Seeking the endgame for Debian’s /usr merge