Here is how you can add SCP500 as a soft dependency. This means your mod will not need to have the mod as a main dependency and will only use SCP500's functionality if it detects the mod exists.

internal class SCP500Compatibility
{
    private static bool? _enabled;

    internal static bool enabled
    {
        get
        {
            if (_enabled == null)
            {
                _enabled = BepInEx.Bootstrap.Chainloader.PluginInfos.ContainsKey("ProjectSCP.SCP500");
            }
            return (bool)_enabled;
        }
    }

    internal static bool IsLocalPlayerAffectedBySCP500
    {
        get
        {
            if (enabled)
            {
                return LocalPlayerAffectedBySCP500();
            }
            return false;
        }
    }

    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.NoOptimization)]
    private static bool LocalPlayerAffectedBySCP500()
    {
        return SCP500.SCP500Controller.LocalPlayerAffectedBySCP500;
    }
}