In etc/system.properties we have:
com.sun.jndi.ldap.connect.pool.protocol="plain ssl"
This results in the system property actually getting set to "plain ssl" (with quotes), you can verify this using the "support/system information" UI
If the quotes are removed the value shows up correctly in the "support/system information" UI, no quotes are present.
I'm pretty sure this means LDAP connection pooling is not currently enabled in Nexus 3m7.