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Is it even okay with the external ad providers or GDPR?

It isn't, but it isn't relevant because GDPR is nowhere near enforced enough for Reddit to get in trouble for this.

When Facebook and Google are dealt with, then Reddit will have to worry and will change course.

So the classic case is happening. If you are powerful enough, you don't need to worry about any regulation.

When it comes to the GDPR you don't even need to be powerful. Even very small players get away with breaching it.