LDAP users in Catalina cannot log in - CRAM-MD5 error in opendirectoryd
Does it work for you if you try to edit the plist file indirectly with odutil, as follows:

odutil set configuration /LDAPv3/foo.example.com module ldap option "Denied SASL Methods" CRAM-MD5 DIGEST-MD5 LOGIN PLAIN NTLM