I'm trying to figure out if it is possible to download Minidumps that were uploaded to sentry (https://sentry.io)?
A Google search leads me to this thread from 2018 https://forum.sentry.io/t/downloading-minidump/3939 which indicates it's not possible for the open-source version. Does this still hold true?
You can. You need to opt-in in the project settings:
Since minidumps contain parts of the memory space of the process that crashed, it could contain some PII data so you must opt-in to be able to store it in Sentry and subsequently be able to download it.