> qca-tls depends on qca--in fact, as far as I can tell, it's a qca plugin.  I assume 
> the same is true of their t* versions.

That is what I thought. The qca package is still built in Slackware releases, albeit, updated to version 2.0.2.

> Do we know which applications are using this, and has anyone tested their
> crypto/SSL capabilities (since this seems to be [t]qca's purpose)?

I know of no tests ever being conducted.

Grepping through the source tree, I find the following use qca.h:

dependencies/tqca-tls/qcaprovider.h:29:#include"qca.h"
tdenetwork/kopete/protocols/jabber/libiris/iris/xmpp-core/securestream.h:24:#include <qca.h>
tdenetwork/kopete/protocols/jabber/libiris/iris/xmpp-core/tlshandler.cpp:24:#include <qca.h>
tdenetwork/kopete/protocols/jabber/libiris/iris/xmpp-core/protocol.cpp:28:#include <qca.h>
tdenetwork/kopete/protocols/jabber/libiris/iris/xmpp-core/stream.cpp:50:#include <qca.h>
tdenetwork/kopete/protocols/jabber/libiris/iris/xmpp-core/simplesasl.cpp:27:#include <qca.h>
tdenetwork/kopete/protocols/jabber/libiris/iris/xmpp-core/connector.cpp:35:#include <qca.h>
tdenetwork/kopete/protocols/jabber/libiris/iris/jabber/s5b.cpp:28:#include <qca.h>
tdenetwork/kopete/protocols/jabber/libiris/qca/src/qcaprovider.h:29:#include "qca.h"
tdenetwork/kopete/protocols/jabber/libiris/qca/src/qca.h:2: * qca.h - TQt Cryptographic Architecture
tdenetwork/kopete/protocols/jabber/libiris/qca/src/qca.cpp:21:#include "qca.h"
tdenetwork/kopete/protocols/jabber/libiris/cutestuff/network/httppoll.cpp:27:#include <qca.h>
tdenetwork/kopete/protocols/jabber/jabberclient.cpp:27:#include <qca.h>
tdenetwork/kopete/protocols/jabber/jabberaccount.cpp:28:#include <qca.h>
tdenetwork/kopete/protocols/jabber/ui/jabberregisteraccount.cpp:36:#include <qca.h>
tdenetwork/kopete/protocols/groupwise/libgroupwise/qcatlshandler.cpp:22:#include <qca.h>
tdenetwork/kopete/protocols/groupwise/libgroupwise/tests/clientstream_test.h:22:#include <qca.h>
tdenetwork/kopete/protocols/groupwise/libgroupwise/securestream.h:24:#include <qca.h>
tdenetwork/kopete/protocols/groupwise/libgroupwise/gwclientstream.h:23:#include <qca.h>
tdenetwork/kopete/protocols/groupwise/libgroupwise/qca/src/qcaprovider.h:29:#include "qca.h"
tdenetwork/kopete/protocols/groupwise/libgroupwise/qca/src/qca.h:2: * qca.h - TQt Cryptographic Architecture
tdenetwork/kopete/protocols/groupwise/libgroupwise/qca/src/qca.cpp:21:#include "qca.h"
tdenetwork/kopete/protocols/groupwise/libgroupwise/gwclientstream.cpp:24:// #include <qca.h>
tdenetwork/kopete/protocols/groupwise/gwaccount.cpp:46:#include <qca.h>

-- 

Darrell