If I subscribe to the current version of Indesign, would I be able to open CS3 files? In the past, I know people had to get CS4, then CS6, etc., to open older files. Hopefully Adobe has corrected this error and madfe it possible to open old files in the current version.
I don't believe you will have a problem opening up earlier version of InDesign files, including CS3. The problem is trying to open up newer InDesign files. I don't think I've heard of any general problems in the past unless they had become corrupted.
I think you are confusing a few issues.
As Steve mentioned, you can open older files.
CS4 is where the .inx format was replaced with .idml exchange format.
CS6 was the last version to open PageMaker files.
Indeed, it happens sometimes that people have problems opening old files with the latest InDesign version. I would recommend to save as inx file and go to someone and aend them the indd and inx file to test, if the latest version can open these files. If not, try to find someone with CS4 to CS6 and make there the test, if it is pssible to open with and ask if they can convert those file to then current indd and idml files.
Here’s an InDesign CS1 file from 2006 opened into CC2021 (v16.4):