See the recommendation at https://github.com/dotnet/designs/blob/main/accepted/2024/net-standard-recommendation.md.

Building for netstandard1.0 using the .NET 9 SDK produces the following warning:

warning NETSDK1215: Targeting .NET Standard prior to 2.0 is no longer recommended. See https://aka.ms/dotnet/dotnet-standard-guidance for more details.

Although this is technically a breaking change, I don't expect this change to affect users in practice.