While it is clearly a shitty move, it's not really clear to me that posts on Reddit consist of personally identifying information as protected by the GDPR.

Every post is tied to a username and email address, making it personal information, since each poster can be identified. I'm sure they're also tracking further metrics such as IP addresses, browser fingerprints, etc. It is immaterial if we from the outside are able to identify users, it only matters if it's possible given the data available to the processor. In this case, it is. Not to mention, there is a good chance texts and posts themselves contain plenty of personal information, such as linking to other user profiles, mentioning and discussing people, etc.

If they were GDPR-compliant before, I don't see how they've changed to not be GDPR-compliant now. They allow people to delete their accounts and their posts if they wish, which removes all identifying information from their system.

Frankly, this looks like just a "I just hate Reddit! There's gotta be something I can hit them with!" flailing attempt to me.

They 'allow' people to delete their posts and accounts...

But never actually delete anything from their databases. I've had years-old comments I deleted mysteriously reappear despite being gone for months.

So contact them about that, then. This doesn't change anything. If they were GDPR-compliant before they're still GDPR-compliant, if they weren't GDPR-compliant then they still aren't. My point is that this AI training stuff has nothing to do with that.