Magnolia 5.4 defines httpclient 4.3.5 as the managed version.

When running Magnolia 5.4.8 with httpclient 4.5, the Magnolia login form is broken (submitting the form with valid or invalid credentials always causes an exception).

When running Magnolia 5.4.8 with httpclient 4.5.2, which currently seems to be the latest, in addition to the login form also the Magnolia page app is broken (double click doesn't open page detail anymore).

Since httpclient is one of Apache's most frequently used frameworks (e.g. used in other common SOAP clients and server libraries, REST, and numerous custom communication solutions), Magnolia should be compatible with a recent version to give other libraries and user code the opportunity to work with the full feature range offered.
As aforementioned bugs indicate, httpclient versions are not fully compatible with each other in aspects that are important to Magnolia, so attempting to support several versions of httpclient alongside may be a difficult task. It might be the better choice to entirely update the managed version to the most recent version of httpclient for Magnolia 5.5.