Clear text formatting in Slides using Apps Script

How about this workaround? I think that there are 2 patterns for this situation. From your question, in this answer, it clears the formats of underline, italic, bold, strikethrough, fontFamily and fontSize. In this answer, "clear" means that it modifies to the default formats.

Workaround 1:

Use Slide services. At first, it retrieves the default values of the text style. As a sample, put a text box including a text value to a slide. In this case, the format of text value is not changed. Using Slides API, the default text style is retrieved as follows.

"style": {
 "underline": false,
 "italic": false,
 "bold": false,
 "strikethrough": false,
 "fontFamily": "Arial",
 "fontSize": {
  "magnitude": 14,
  "unit": "PT"
 }
}

In this workaround, these values are used as the default values. The sample script is as follows.

Sample script:

In this sample, the text styles of the PageElementType of SHAPE and TABLE are modified to the default formats.

function toDefault(text) {
  if (text.getRange(0,1).asString().charCodeAt(0) != 10) {
    var style = text.getTextStyle();
    return style.setUnderline(false).setItalic(false).setBold(false).setStrikethrough(false).setFontFamily("Arial").setFontSize(14);
  }
  return null;
}

function myFunction() {
  var s = SlidesApp.getActivePresentation();
  var slide = s.getSlides()[0]; // As a sample, 1st page is used.
  var pageElements = slide.getPageElements();
  pageElements.forEach(function(e) {
    if (e.getPageElementType() == "SHAPE") {
      var text = e.asShape().getText(); 
      toDefault(text);
    } else if (e.getPageElementType() == "TABLE") {
      var table = e.asTable();
      for (var row = 0; row < table.getNumRows(); row++) {
        for (var col = 0; col < table.getNumColumns(); col++) {
          var text = table.getCell(row, col).getText();
          toDefault(text);
        }
      }
    }
  });
}

Workaround 2:

Use Slides API. For updateTextStyle of Slides.Presentations.batchUpdate(), when only "fields": "underline,italic,bold,strikethrough,fontFamily,fontSize" without setting each value is used, the default values of underline,italic,bold,strikethrough,fontFamily,fontSize are used. In this workaround, this is used.

Sample script:

In this sample, the text styles of the PageElementType of SHAPE and TABLE are modified to the default formats.

function myFunction() {
  var s = SlidesApp.getActivePresentation();
  var slide = s.getSlides()[0];
  var presentationId = s.getId();
  var pageElements = slide.getPageElements();
  var reqs = pageElements.reduce(function(o, e) {
    if (e.getPageElementType() == "SHAPE") {
      if (e.asShape().getText().getRange(0,1).asString().charCodeAt(0) != 10) {
        o.push({"updateTextStyle": {"objectId": e.getObjectId(), "fields": "underline,italic,bold,strikethrough,fontFamily,fontSize"}});
      }
    } else if (e.getPageElementType() == "TABLE") {
      var table = e.asTable();
      var objectId = e.getObjectId();
      for (var row = 0; row < table.getNumRows(); row++) {
        for (var col = 0; col < table.getNumColumns(); col++) {
          var text = table.getCell(row, col).getText();
          if (text.getRange(0,1).asString().charCodeAt(0) != 10) {
            o.push({"updateTextStyle": {"objectId": e.getObjectId(), "cellLocation": {"columnIndex": row, "rowIndex": col}, "fields": "underline,italic,bold,strikethrough,fontFamily,fontSize"}});
          }
        }
      }
    }
    return o;
  }, []);
  var resource = {"requests": reqs};
  Slides.Presentations.batchUpdate(resource, presentationId);
}

Note:

• When you use workaround 2, please enable Slides API at Advanced Google Services and API console.
• If you want to clear all formats of the text style, for the workaround 2, please modify from "fields": "underline,italic,bold,strikethrough,fontFamily,fontSize" to "fields": "*".

References:

• Class TextStyle
• UpdateTextStyleRequest