Her death was purposefully ambiguous...really just like Star Lord's and Nova's.
She even had a monument along side theirs at the end of Thanos Imperative.
We only had Mantis' word to go off, based on what she saw....probably some barely recognizable chard remains.
I mentioned this a while back on the old CBR forum here but a few years ago I got to hang with GotG artist Brad Walker and he sort of alluded that DnA had plans for all of the recently "dead" (at the time) Guardians. The plan seemed to be that they were going to lay the seeds for their return in Annihilators; Remember it wasn't just Phy who died, and was forgotten, most of the DnA team died before/during Thanos Imperative. Pete, Drax, Rich, Phy and Adam were all planned to make a return in some manner or another, it's just the plan was side tracked due the moves in creative. In Phy's case it seemed that although her mortal form as Martyr was destroyed her body as it was BEFORE turning into Martyr was still in possession of Oblivion. That tangent was and maybe/ HOPEFULLY is still the key to her return...I just don't see some one as stubborn as Moondragon NOT going to hell and back to restore Phy.
One thing is certain, Phyla Vell (whom I think should just stay named Phyla Vell, sort of like how Jean Grey and Kitty Pryde outgrew their need for code-names) is not completely forgotten. My costuming team all met James Gunn recently (@ Dragoncon) we were all decked out in our home built GotG costumes (Annihilation era of course), we had a Phyla and Moondragon with us and as he looked us over saying "Hmmmm, now which one of you guys are gonna be in the sequel..." he pointed to our obvious group members Star Lord (me), Rocket, Groot, etc. But when he got to our Phy and Moondragon he said "You guys I'm still trying to figure out." Then he continued by saying to our Bug cosplayer "Bug! Man I'm sorry but I can't use you. We don't own you. I really tried to get you in!" Which crushed our buddy cosplaying him, lol. Anyway, the point is that Gunn has her in mind and if the theme of GotG 2 is all about confrontations between fractured and estranged families: Star Lord/his dad, Gamora/Nebula/Thanos, & Drax/Moondragon, then there's a strong possibility Phy will show up in the sequel and with that a renewed focus in the 616.
With the Drax and Moondragon relationship in particular they can easily tie in Phyla if they choose to...she even has her own heavy Family issues..a heroic dad who casts a HUGE shadow and a bat-shit older Brother. So much potential really, and that's not even scratching the surface, the fact that she'd be the first openly lesbian hero to feature prominently in a blockbuster DISNEY film is a pretty big deal.
Also, if that isn't enough to convince you Phyla is not TOTALLY forgotten, She is actually a playable character in the New Guardians of the Galaxy Mobile game. That little gem is written by none other than Dan Abnett. You know HE won't let Phyla fade away, he LOVES her.