Yeah, this sort of thing is like a pessimistic case for Cassandra and various databases that are designed to model data as an immutable set of facts and to model deletions as retractions or the like.

Apparently it defaults to 10 days for tombstone purging and recommends not going below 5 days. How bad is performance actually going to be at a nice slow several-day compaction rate?

The pessimistic case sounds like trying to remove things within hours.

All I can say is that not everyone's situation is the same. If you have a small forum where a few hundred people post a few dozen messages a day, it obviously won't be a big deal. There are situations where the amount of generated information is much larger than that. Webserver logs are one possible example.

It isn't an impossible problem to solve, but the GDPR is a significant time and a money burden that will especially be an issue for small startups that don't have millions in venture funding to spend on this.