In Skyrim Classic/Legendary Edition ("oldrim"), you can make dialogue unskippable by modifying dialoguemenu.swf directly instead of using the SKSE plugin. The only disadvantage is that you'll have to do it again every time you use a different mod for the dialogue menu UI.

Download and install JPEXS Free Flash Decompiler (FFDec) and use it to open dialoguemenu.swf. 
In the tag list, navigate to scripts/__Packages/<default package>/DialogueMenu. 
Click the Edit ActionScript button at the bottom of the screen.

Change
function SkipText()
{
   if(this.bAllowProgress)
   {
      gfx.io.GameDelegate.call("SkipText",[]);
      this.bAllowProgress = false;
   }
}
to
function SkipText()
{
if(this.TopicList.EntriesA.length == 0)
{
   gfx.io.GameDelegate.call("SkipText",[]);
}
}

Change
function onCancelPress()
{
if(this.eMenuState == DialogueMenu.SHOW_GREETING)
{
this.SkipText();
}
else
{
this.StartHideMenu();
}
}
to
function onCancelPress()
{
this.StartHideMenu();
}

Click the save-button at the bottom of the screen, then save the file (you may want to save as a separate file to keep the original intact).