extension_drop depends on cat_tools, which already requires PostgreSQL 12+. A fresh install of extension_drop was already broken below that too (a cat_tools function it calls needs PG11+ internally) — this just makes the declared floor honest.
CREATE EXTENSION already raises client_min_messages to WARNING for
the install script (only-raising, so a stricter caller is respected) and
restores it afterward, so the script’s own save/restore was redundant — and being blind to the caller’s level, it could actually lower a
stricter caller’s client_min_messages during install.
No functional issues turned up in the 9+ years since 0.1.1; promoting
from testing to stable.