Thank you for the response. I'm working with Vitaly L, and I can provide some update on this.
1) Your suggestion works. If I remove the existing project file and generate the code for .NET Standard, the new project file looks nice and plain.
<?xml version="1.0" encoding="utf-8"?>
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
<AssemblyName>Data</AssemblyName>
<RootNamespace>Data</RootNamespace>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="SD.LLBLGen.Pro.ORMSupportClasses" Version="5.6.0" />
</ItemGroup>
</Project>
2) Our case is a bit more complicated. First, we're still on .NET Framework 4.8. Second, there's some additional info in our Data.csproj file (properties, references, includes) that we'd like to keep on every update of the generated code. I played with the code generation a bit and found out that the only crucial part is this one
<PropertyGroup>
<ProjectGuid>{E9DB475C-34FF-496E-A174-0E9D169BBB67}</ProjectGuid>
</PropertyGroup>
If ProjectGuid property is present, the Designer treats the project file like being in old format, despite of <Project Sdk="Microsoft.NET.Sdk"> tag and .NET Standard 2.0 platform selected, and it adds <Compile Include=...> records for the generated files.
If ProjectGuid property is removed, the code generation works as expected, even for .NET Framework 4.8 platform.
Btw, we're using version "5.6 (5.6.1) RTM".
I think, this one is solved. Thank you again.