To save 200MB of RAM and reduce CPU spikes, modders almost always remove Windows Defender. While third-party AV (like Bitdefender Free) can be installed, many Ultralight builds break Windows Security Center entirely. You are running bare metal without an integrated antivirus.
For those who find the official ISO too heavy but fear the risks of third-party builds, there is a middle ground. Tools like windows 10 ultralight 22h2 190455198iso better