This was also an issue with Discord - Discord still doesn't (by Apple didact) permit some servers with adult content in the iOS app, because Apple said there was no way to do so and remain in the store.
Given that the problem with Tumblr was the accessibility of CSAM, I think Apple are probably on the right side here saying "just marking it 18+ is not ok" since, y'know, the issue wasn't that "CSAM is available to minors" but "CSAM is available to anyone".
I think Tumblr was singled out and made an example of because they had a narrower user demographic spread, particularly popular with young women. In cynical business logic, this made them a safer target for bullying than a site with broader appeal like reddit, facebook or twitter.
"Additionally, a subset of age-restricted servers that are specifically focused on explicit pornographic content will be blocked entirely on iOS. "