The XAxis.MajorGridLines of the chart YAxis are always visible as of 2020.1.114
Code to reproduce the issue:
<telerik:RadHtmlChart ID="RequestsMonthChart" runat="server" Height="500">
<ClientEvents OnLoad="chartLoad" />
<PlotArea>
<Series>
<telerik:ColumnSeries Name="Series 1">
<SeriesItems>
<telerik:CategorySeriesItem Y="30" />
<telerik:CategorySeriesItem Y="10" />
<telerik:CategorySeriesItem Y="20" />
</SeriesItems>
</telerik:ColumnSeries>
</Series>
<XAxis Name="Year" DataLabelsField="Month">
<LabelsAppearance RotationAngle="-35" />
<MajorGridLines Visible="false" />
<MinorGridLines Visible="false" />
<Items>
<telerik:AxisItem LabelText="Item 1" />
<telerik:AxisItem LabelText="Item 2" />
<telerik:AxisItem LabelText="Item 3" />
</Items>
</XAxis>
<YAxis>
<MajorGridLines Visible="false" />
<MinorGridLines Visible="false" />
</YAxis>
</PlotArea>
<Legend>
<Appearance Visible="true" Position="Bottom" Width="500" />
</Legend>
<ChartTitle Text="Requests Received by Month" />
</telerik:RadHtmlChart>