seal-single-value-enum-by-default in autorest.md is a temporary flag. When it stops working, enums with single value will no longer be treated as constants. To avoid this, add x-ms-enum.modelAsString=false to all enums that should remain constants, e.g.: e.g.:
https://github.com/Azure/autorest.csharp/blob/25d9b624ae1e6c5faf65fbb8176e718f85c6b4c8/test/TestProjects/BodyAndPath-LowLevel/BodyAndPath-LowLevel.json#L106 (in this example, however, parameter is still created cause EnumName2 has 2 values).