Is it still possible to download Adobe InDesign 2018 in 2021? My version is 16.4, I need 13.1/2!

Do you have a compatible computer OS that can support older versions of ID, e.g macOS El Capitan to Mojave?  See below for 2018 direct downloads.  Be sure to follow the very important instructions or it won't work. 

https://prodesigntools.com/adobe-cc-2018-direct-download-links.html

Hope that helps.

I do not know how far back this process will go, but try...
Direct Download Links https://prodesigntools.com/tag/ddl MAY help
-you MUST do what it says in Note: Very Important Instructions
-https://prodesigntools.com/adobe-cc-2015-direct-download-links.html#download-instructions
-the prodesign links go directly to Adobe's file server, so this process will
--only work until Adobe completely removes the old version(s) from the server
.
IF/When you do have a download, be sure to copy it to AT LEAST one external device
-since Adobe does sometimes remove old versions from the server
-a usb flash drive or external hard drive, or write to a DVD