I've been thinking about this question for a while and periodically looking for new information but have yet to find anything explicit on this. However, I think we can assume she was Snapped and so only came back at the end of Avengers: Endgame after the 5 year gap.
In the post credits scene for Ant-Man and the Wasp we see Scott, Janet, Hank and Hope setting up the machine to jump into the Quantum Realm and Scott going in. Janet, Hank and Hope are then Snapped leaving Scott trapped in the Quantum Realm. Scott is later released after the 5 year gap by a rat stepping on a button and nothing intentional. However, they were gathering the quantum particles for Ava and so she and Bill would likely have known where they were. It stands to reason then that after the Snap had they survived they would have gone to try and see what happened and brought Scott out so the particles could be used to help Ava. As this didn't happen I think we can assume they were both Snapped.
However, even if they weren't Snapped I would argue that she would still have survived. Janet already healed her when she came out of the Quantum Realm and they aren't in a rush in the post credits scene meaning it isn't exactly a time sensitive issue like it was before. She originally only had around two weeks to live and Janet seems to have stabilised her.
Ava: How long have I got?
Bill: A couple of weeks, maybe.
Ant-Man and the Wasp
Ghost seems to have survived 20+ years by the time of Ant-Man and the Wasp when her time has all but run out and then is stabilised by Janet. This means that if what Janet did was really powerful and "reset" her she could survive another 20+ years and they were only going to get the particles just to help maintain her condition and so she wouldn't be in constant pain rather than trying to heal her completely.
Now for some speculation on future events. I don't know much about the Roger Wardell twitter account other than it went viral for supposedly predicting some events that happened in upcoming films. I'm not sure if it is an accurate source for leaks or if it was like those World Cup accounts that predicted every outcome and then deleted the ones that didn't happen making it look like they knew the future.
That said the account has tweeted that there is a Thunderbolts film in the works and that it will feature Ghost. However, Marvel have yet to comment on this from what I can tell. But their silence on commenting about Ghost at all could be taken as evidence that there is something in the works for her depending on how you look at it.
@RogerWardell: Don't worry, Marvel still has big plans for Zemo, Ghost, Justin Hammer, General Ross, and the Abomimation. The Leader is being discussed to return as the main villain of Thunderbolts, which is being planned as a trilogy.
Hannah John-Kamen, Ghost's actress, has also stated that she'd be up for a Thunderbolts movie if one were to come out so she certainly hasn't ruled out the possibility of a return for Ghost.
That would be fun! That would be fun. I think, you know, as well, in the original comics Ghost was with Iron Man, it was actually originally Iron Man (villain), you know, and I actually went when I was in Atlanta, went actually bought the Thunderbolts comic books. Um, I mean with any Marvel character whatever happens to them, you always go the possibility is endless. And with me it would be an honor to work with any of the heroes, and any of the antagonist, and any of the things in the Marvel Universe. I mean, yeah, that would be amazing!?
Screen Rant, Ant-Man & The Wasp Star Would Love To Do A Thunderbolts Movie