I am not sure what ruler/measuring that you are talking about, but it seems you are talking about
Advanced Digitizing Toolbar because this is the only tool that I can see measuring or values to enter there.
If you are looking for
Advanced Digitizing Toolbar, it exists in both QGIS 3.4 and 3.8 versions, but maybe not shown clearly. Right-click on the toolbars above the canvas and under Toolbars, select
Advanced Digitizing Toolbar
Advanced Digitizing Toolbar will be ONLY if the data that you are working is projected which means a CRS that uses meter or feet units. If you working with geographic data the toolbar will not be activated and you cannot see the measurements.