I have been using this code successfully for a long time:

function demo() {
   SpreadsheetApp.getUi().showModalDialog(HtmlService.createHtmlOutput('<b>Patience!</b>'), 'Processing! Please wait');

//do some stuff

   SpreadsheetApp.getUi().showModalDialog(HtmlService.createHtmlOutput('<script>google.script.host.close();</script>'), 'Ready. Please continue');  
}

Since yesterday (26.07.2024) this code does not work as before which is described very well in this thread.

Hence, does anybody know if this is a permanent change or if it is a temporary glitch?
Is there any documentation anywhere describing this new behaviour?

My GAS skills are very basic and I do not understand how the solution/work around in the thread above can be applied to me since I am only using Google Sheets.

Back in the day I used to use this approach, but it sucked of course:

SpreadsheetApp.getActive().toast("Processing...", "Please WAIT!", 60); 

Since my productive application stopped running – i.e. the end users do not know how to cope – because of this change/bug I am a bit desperate and looking for a quick fix to solve this.