The short answer is that it's not possible to delete your posts or other data from Manyverse. This is due to design choices deep inside the SSB protocol and because of how data spreads in an off grid social network.
It's hard to remove your data from the internet, no matter what the platform is, because people can keep copies of it. For instance, it's common to keep screenshots of posts on Twitter or Facebook, that prevents the author from retracting what they said even if they delete their post. However, deletion on those centralized platforms is still useful, and helps the author gain confidence that others don't have copies of their data.
On SSB, deletion is more complicated because the social network is by design a network of data copies. When you view your friends' posts, you are actually viewing a copy of their posts. So if your friend were to delete their post, in practice they would have to issue a request for everyone else to also delete copies of that post. These deletion requests could be built in an automated way — and indeed the SSB protocol is being redesigned to allow deletes in the future — but the propagation of requests is made more difficult in an truly off grid scenario.
For instance, suppose Alice is friends with Bob, and Bob posts something new. Alice receives a copy of that post, reads it, and keeps it. Then, Alice leaves to spend months in Africa off grid (without internet), while Bob remains in the city with internet. As soon as Alice arrives in Africa, Bob suddenly regrets posting his latest message, so he wishes to delete it. However, he has no way to reach Alice and inform her to delete her copy of the post. Alice, in Africa, further propagates Bob's post to a large community of people using Bluetooth and Wi-Fi, without internet, and Alice is oblivious to the fact that Bob does not want that post shared. There is no way Alice can know and respect Bob's wishes for deletion, and the message can keep propagating for months.
So, while we intend to implement deletion of posts in Manyverse as soon as SSB supports it, we cannot truly support instant global deletion if we assume the off grid use case. Instead, we want to inform users that any content they post on Manyverse can potentially be propagated indefinitely. We think this is a more honest approach that educates the real constraints of eventual communication instead of promising the impossible. On the flip side, posting messages knowing that they can be permanent has a good social effect of posting only non-regrettable content.