Please, I need to download InDesign version 16.4.3

When saving for backwards compatibility just make sure not use any features that are not available in earlier versions. James Wamser has put together a great resource to see what has been added/removed in all InDesign versions since 1999: InDesign New Feature Guide

You may want to consider sharing files with your client using InDesign CS4 or later IDML (.idml) format. Creating those files are only a File>Save As menu command away, and a lot easier on everyone than hunting down old software and installing it over/with/instead of your current software configuration.

Rather than scale your application set back to accommodate your client, you may be able to get away with only scaling back your file format, and everybody can work with the version of InDesign they're currently using. This isn't exactly an answer to the question you asked, but it could be an answer that will work well for you and your client anyway.

Hey, it's worth a shot ...

Randy

Once you get the 16.0 version installed you can copy the program files from a backup copy or another computer that has the patches installed and you will be properly updated, but there is no other way I know of to get the patches.

Adobe normally only provides links on the Creative Cloud app to the two most recent versions. If you had a good reason to get an earlier version (but only the initial build, like 16.0 for CC 2021), you can request a link from a customer service representative.  Use the link below to contact customer service after logging into the Adobe ID for your account:

https://helpx.adobe.com/contact.html