Tumblr is just one example. Discord had to restrict all NSFW channels in their app for iOS users because their randomly assigned App Store reviewer managed to get their account into an NSFW channel and found the expected.
Reddit just never had to deal with this issue because they didn't even have an iPhone app for years (their current app is a highly modified version of an older app, Alien Blue, but AB was pulled from the App Store when Reddit bought it) or they deal with it quietly so you don't hear about it. Not all companies who get shafted on the NSFW rule speak up about it, but those that have... all have repeatedly commented on the fact that the process is basically completely opaque and you need to basically pray that Apple doesn't decide to pull your current version either.
Tumblr picked the easy way out (ban all NSFW in general) because they were about to be sold to Verizon and Yahoo didn't want to bother trying to find a way to appease to apple at the last minute.
In a way though, that means Safari should be banned too. And the camera app, I mean point it down in the right state of undress... ;)
From what I've heard the Apple App Store isn't much better in this regard, except there you just don't have the option to ignore it (and the jailbreaking community seems more open, except the same "pay for every tiny thing" culture that plagues iOS has made it way over to that platform).
The EU forcing Apple to open up iOS should ideally result in a more fair, better curated ecosystem of smaller app stores and the development of apps that people actually want to use rather than apps that exist to make exciting number go up at an ad agency. There'll be some initial blowback, but the long-term benefits will be that much better for mobile phones on the whole (which to be frank, is desperately needed).
It also gives a secondary avenue for apps against Apple's app store guidelines, which have been noted in the past to basically be completely arbitrary (the main examples of this are the fact that Apple plays censor for VPNs in some countries, Steve Jobs telling people that want to have NSFW apps on their iPhone to "go buy an android", the last policy still leading to numerous social media apps getting in trouble because an app reviewer found something NSFW (this is why Tumblr banned NSFW) and for a very specific example of a game that somehow kept violating the guidelines: the iOS port of the Binding of Isaac was rejected for over a year because Apple wanted to play moral guardian, there's also the completely unreasonable ban on allowing emulation and really I could continue on and on about how absurd this is).