How to release ports on the SSH server when a reverse ssh tunnel disconnects abruptly/uncleanly?

You have to use ClientAliveInterval in your sshd_config.

ClientAliveInterval 15

Ref: man sshd_config

ClientAliveCountMax
         Sets the number of client alive messages (see below) which may be
         sent without sshd(8) receiving any messages back from the client.
         If this threshold is reached while client alive messages are
         being sent, sshd will disconnect the client, terminating the
         session.  It is important to note that the use of client alive
         messages is very different from TCPKeepAlive (below).  The client
         alive messages are sent through the encrypted channel and
         therefore will not be spoofable.  The TCP keepalive option
         enabled by TCPKeepAlive is spoofable.  The client alive mechanism
         is valuable when the client or server depend on knowing when a
         connection has become inactive.

         The default value is 3.  If ClientAliveInterval (see below) is
         set to 15, and ClientAliveCountMax is left at the default,
         unresponsive SSH clients will be disconnected after approximately
         45 seconds.  This option applies to protocol version 2 only.

 ClientAliveInterval
         Sets a timeout interval in seconds after which if no data has
         been received from the client, sshd(8) will send a message
         through the encrypted channel to request a response from the
         client.  The default is 0, indicating that these messages will
         not be sent to the client.  This option applies to protocol
         version 2 only.