Platform instance claim is an optional claim in LTI 1.3 specification. See https://www.imsglobal.org/spec/lti/v1p3#platform-instance-claim.

In mod_lti implementation, when configuring a pre-defined tool, the last section of the configuration allows setting the "Default organisation ID", "Organisation ID", and "Organisation URL", which are to be used to construct the variables in this claim  in function lti_build_standard_message(), with $orgid to be the guid of the platform instance guid. This is at https://github.com/moodle/moodle/blob/0360329c46ad4c1d20e84268d6507f0ea29cc2f0/mod/lti/locallib.php#L983-L1019

However, when launching a LTI 1.3 tool, function lti_initiate_login() is called, which in turn get request parameters from function lti_build_login_request() at https://github.com/moodle/moodle/blob/0360329c46ad4c1d20e84268d6507f0ea29cc2f0/mod/lti/locallib.php#L3496-L3524

and it doesn't return the tool_consumer_instance_guid parameter.

As a result, the platform claims are never sent in the launch requests for LTI 1.3 external tools.