This is a DNSSEC test zone signed only with ML-DSA-44 (algorithm 18, per draft-westerbaan-dnssec-mldsa).
If your resolver validates DNSSEC and supports algorithm 18, this zone is secure. A validating resolver without ML-DSA-44 support treats it as insecure (unknown algorithm).
Try: dig +dnssec only.alg18.westerbaan.name A
Related test zones: dual, downgrade, upgrade and broken13; see the overview.