We are currently using AEM6.3 and have a page which has extjs dialog appearing on html page in preview mode in Author.
As most of the things are now supported on touch ui and we are moving away from classic.
Wanted inputs as till what upgraded version of AEM the extjs library will be supported and till when can we keep using classic dialog and extjs based dialog related pages
Check the first item of the list of deprecated items of AEM 6.4 .
 Deprecated features