In at least tickling and shinyyiff there is no easy way to toggle or set nsfw using the default web interface. The checkbox either doesn’t load, or disappears immediately. For creating the post, nsfw=true can be manually added as a variable. I’d have to dig into the api to see if there is an easy way to toggle a post that has already been created. I don’t know if this issue is a problem on apps or other front ends.
I believe there was a change to some Lemmy communities where if the community itself is marked as “NSFW” there’s no ability to toggle it off anymore: https://github.com/LemmyNet/lemmy-ui/pull/2887 (https://join-lemmy.org/news/2025-04-08_-_Lemmy_Release_v0.19.11).
If the post isn’t flagging automatically as NSFW in a NSFW community, then there may be a backend bug that needs to be reported to the Lemmy developers via GitHub.
The property itself still seems toggle-able with the api. The changelog and pull linked seem to be only on the default front end. I don’t see an associated change with the back-end which is mentioned in the PR. If web tools are enabled, the toggle shows up for creating a post. If you add nsfw=true as a parameter in the submission URL it is marked appropriately, but it is not set by default. https://lemmynsfw.com/post/23232239 (This seems to have caused issues elsewhere too).
After playing around with it. the posts don’t show up unless nsfw is enabled, but if you have nsfw enabled the post does not have the visible nsfw marking. If you don’t blur NSFW posts, there is no visible way to know it is NSFW
TL;DR It is treated as NSFW, but it is not visibly marked as NSFW.