_SSLProtocolTransport keeps reference to protocol after close. It leads to reference cyeles between the transport and the protocol and is bad for gc. Clearing this is better as it frees up the memory immediately without waiting for the gc. This causes memory leaks in some cases as if an exception occurs the deallocation is delayed even further as tracebacks keeps ref to frame and frame keeps locals alive.

Linked PRs

• gh-98539: Make _SSLTransportProtocol.abort() safe to call when closed #104474
• [3.11] gh-98539: Make _SSLTransportProtocol.abort() safe to call when closed (GH-104474) #104485