Yes, MAAS 2.3 (the last revision of MAAS supported on Xenial) can support iSCSI when it is placed in backward compatibility mode; how to do this is documented in the changelog as follows:
maas $PROFILE maas set-config name=http_boot value=False
MAAS 2.2 and earlier (no longer supported) used iSCSI by default.
So you could test this on any version of MAAS on Xenial.
Yes, MAAS 2.3 (the last revision of MAAS supported on Xenial) can support iSCSI when it is placed in backward compatibility mode; how to do this is documented in the changelog as follows:
maas $PROFILE maas set-config name=http_boot value=False
MAAS 2.2 and earlier (no longer supported) used iSCSI by default.
So you could test this on any version of MAAS on Xenial.