Populate dropdown in cell with list of tabs?

Add this function to the script editor and save the project:

function get_sheets() {
  return SpreadsheetApp.getActiveSpreadsheet().getSheets().map(function(x) {return x.getName();});}

Then you can add it to a cell. Here I select A2 as the dropdown formular. I simply select the range of column B.

And voila:

Go to Tools > Script editor

Paste into Code.gs:

/*** return sheet names for active document
 * @customfunction
 */
function sheetnms() {
  return SpreadsheetApp.getActiveSpreadsheet().getSheets().map(function(x) {return x.getName();});
}

Save as a project.

Go back the Google sheet.

Enter =sheetnms() into a cell.

When creating a data validation, get the list from the range of cells with the sheet names.

Note: there may be some delay for the list to update.