Public IP basic SKU - What's the actual risk?

Public IP addresses allow Internet resources to communicate inbound to Azure resources.Inbound communication from the Internet to the resource, such as Azure Virtual Machines (VM), Azure Application Gateways, Azure Load Balancers, Azure VPN Gateways, and others.

Ref: how Azure Public IP completely maps to the Azure Security Benchmark, see the full Azure Public IP security baseline mapping file. https://github.com/MicrosoftDocs/SecurityBenchmarks/raw/master/Azure%20Offer%20Security%20Baselines/1.1/azure-public-ip-security-baseline-v1.1.xlsx

In terms of Security aspects, Standard Public SKU is Secure by default model and be closed to inbound traffic when used as a frontend. Allow traffic with network security group (NSG) is required (for example, on the NIC of a virtual machine with a Standard SKU Public IP attached)., Whereas the Basic SKU is Open by default. Network security groups are recommended but optional for restricting inbound or outbound traffic.