Add missing argument (#3023)

* Add missing argument

* chaneglog

* changelog

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## Unreleased
+
+### Fixed
+
+* Respect the `http1` argument while configuring proxy transports. (#3023)
+
 ## 0.26.0 (20th December, 2023)
 
 ### Added

httpx/_client.py

@@ -1493,6 +1493,7 @@ class AsyncClient(BaseClient):
         return AsyncHTTPTransport(
             verify=verify,
             cert=cert,
+            http1=http1,
             http2=http2,
             limits=limits,
             trust_env=trust_env,