I'm assuming she gave more advanced notice than yesterday because they're probably done shooting now, right? Or, at the very least, done writing. Again, with the flash-forwards, we know that Felicity stays in that cabin outside of town with her daughter. If those flash forwards are canon, there's the reason for her to be gone.
One interesting question will be what time period an eighth season would take place / where season 7 ends. Are we going to end a month or so from where we are now? Flash forward to the cabin after Mia is born? Or, as someone online suggested, maybe the 8th season takes place entirely in the future with the future Team Arrow. Unless that storyline is resolved, it's something they could do. I mean, it didn't work on Fringe, but maybe it can work here.
I don't know a ton about it because, frankly, it's all quite exhausting to follow all these sects of Arrowverse fandom. Just like there were Felicity and Laurel sects, there are also Caitlin and Iris sects. So whenever I tried to go down the rabbit hole, everything is tainted with some form of bias, and it's hard to tell what's true and what's rumor based on assumptions from a comment that may or may not have been innocuous. So take all this with as much salt as you'd like.
Candice - As you might know, racist assholes have been bothering Candice for her entire run on the Flash. Whether they don't like the race-swapping or the interracial aspect, she's had to deal with a bunch of crap in what was supposed to be a fairly big break. While she's taken it in stride, some feel that she's bothered by Iris' lack of a clear role or lack of any storyline that she can identify with. Caitlin fans say that she can't act, and that Caitlin (despite not being the romantic lead) is the female lead of the show. There was also some drama with Danielle Panabaker behind the scenes that I don't know anything about. She apparently made some sort of comment that made it seem like she was the only female in the show, but I couldn't find too much about it when I just scanned twitter.
There's been rumors that one or both of the actresses are leaving the Flash. And rumors that Carlos Valdes is leaving. And accusations that Grant Gustin is phoning in his performances.
Katie - The only problems with her stem from her getting killed off originally. I know she wasn't happy about that. What's strange is that she had that contract that was supposed to let her appear on all the shows, but I'm not sure she ever appeared on Legends after that and only appeared on Flash once. There's apparently rumors that she might go to Legends next season.
Emily - The only thing I know is the thing she mentioned that she didn't want to see a pregnancy storyline. From a TV Guide interview:
"I don't know if I would be interested in exploring that storyline right now with Felicity," Rickards said at the time. "I feel like she deserves a little something more than that, and I don't want that to be taken the wrong way. I'd just be interested in seeing her, I don't know, like, deal with a villain face-to-face for a consistent number of episodes."
So maybe the writers were going to make her more of a "mom" character (or worse, someone who abandons her baby to do "more fun" stuff) and that's not what she wanted to do. So she thought if that's what they want her to do, Felicity can be that offscreen.