post #4134195 is a good example of what this tag would be(was, previously at some point) useful for. We can tell that sex is happening by context and sound, but we can't directly see it. So why is the tag invalidated?

Probably because the tag is too vague and not very useful. What you are describing seems more like offscreen sex, which is already on that post anyway.

bleph said:
post #4134195 is a good example of what this tag would be(was, previously at some point) useful for. We can tell that sex is happening by context and sound, but we can't directly see it. So why is the tag invalidated?

Use obscured_sex.

Offscreen_sex is for when characters are entirely offscreen and having sex or they are partially on screen but the sex part (penetration/handjob/etc.) is offscreen. There is also obscured_sex for when the characters are mostly/entirely on screen, but the sex part (penetration/handjob/etc.) is obscured from view. There are some unofficial subtags of obscured_sex such as obscured_oral and obscured_anal, but they lack implications. For masturbation, there is a corresponding obscured_masturbation tag, but offscreen_masturbation was invalidated in topic #24415.

There was another discussion about the implied_* tags 2 months ago: topic #39813.

I thought implied required it being mentioned or kind of obvious? In latter case, we use obscured, right? Former, it probably would have been better if wording implied it was specifically being mentioned.

crocogator said:
There was another discussion about the implied_* tags 2 months ago: topic #39813.

Yeah, b ut I do agree with that tag being invalid since others that are similar enough exist, realiasing it wouldn't be a bad idea imo.
Admins have gotten fed up with every tag ending up aliased to invalid_tag anyways