Unity compile settings warning after December 11 update

I recently updated to the December 11 update of Rider 2020.3 (Build #RD-203.5981.141) and now I get the following "Incorrectly accessing old script compilation settings on newer Unity" message in Unity after every script compile. I am currently on Unity 2018.4.14f1 on Windows 10. I tried deleting the plugin from my project and letting JetBrains reinstall it, but no luck. Is there something else I need to do, or is Unity 2018 no longer supported? (We have plans to upgrade our Unity soon, but we're not quite there yet.)

Here's the full message from the Unity console window.

Incorrectly accessing old script compilation settings on newer Unity. Use EditorPrefsWrapper.ScriptChangedDuringPlayOptions
UnityEngine.Debug:Log(Object)
JetBrains.Rider.Unity.Editor.PluginSettings:get_AssemblyReloadSettings()
JetBrains.Rider.Unity.Editor.<>c:<SetupAssemblyReloadEvents>b__24_1(Object, EventArgs)
System.AppDomain:DoDomainUnload()