Yes, you can still find the option to install apps from unknown sources on the Fire TV Stick 4K Max in developer mode. Although Amazon has hidden this menu in recent updates, it's still accessible by following the appropriate steps.

To access it, you first need to enable developer mode by going to Settings > My Fire TV > About, then press the centre button on your remote 7 times. Once developer mode has been activated, you'll see a new menu called 'Developer Options'. It's in this menu that you'll find the 'Applications from unknown sources' option, which you can activate to install external APKs.

Yes, the option is still there on the 4K Max, but Amazon recently hid it. To make it appear, go to Settings > My Fire TV > About. Then click 7 times in a row on the name of your Fire TV Stick.
A message will tell you that you are a "Developer". If you go back, the "Options for developers" menu will appear and you will be able to activate the installation of apps from unknown sources.