If you forgot about Heath during the nine episodes he was absent from The Walking Dead, don't worry: he's already out of sight and out of mind once again. Heath vanished just as quickly as he returned, presumably because the actor who played him, Corey Hawkins, just landed the lead role in 24: Legacy. Heath wasn't in a good place in his brief return to the screen; he seemed almost entirely hopeless, left feeling lost and miserable after taking part in the murder of the Saviors at the satellite station. Soon after that little update, Heath wasn't seen again for the rest of the episode. So what happened to Heath on The Walking Dead?
While searching a settlement on a bridge for anything useful (there were a lot of bullets to be found before they were covered in a zombie avalanche), Heath and Tara were overrun by walkers and forced to separate. Tara ended up in the water, but Heath's fate was less clear. By the time Tara made it back to the bridge, Heath was long gone with only a few clues left behind: his glasses, a keycard with "PPP" written on it, and a missing RV. There was no concrete sign of Heath.
The crushed glasses left behind indicate a struggle. Heath could have lost them while fighting off walkers, but it's also possible a much more human threat got to him. The Saviors have been causing trouble just about everywhere you look in Season 7 and they're familiar with the sight of an Alexandrian RV, so they could have kidnapped Heath like they did Daryl. The keycard could point to the Saviors as well, because the buildings they seem to frequent (satellite station, abandoned factory) are the kind that would feasibly utilize keycards.
If they did take Heath, that would actually fit with a storyline from the comics about an Alexandria resident named Holly. She was imprisoned by Negan but later returned to Alexandria, where it was revealed that she had become a walker. A bite from her ended up killing Dr. Denise (who is long dead on the show). If Heath took on that storyline, it would keep him out of the way while Hawkins films his other projects and also provide a painful surprise upon his return, which is just the kind of thing the show likes to do.
The Walking Dead chose not to kill Heath onscreen, which means the door is open for his return at some point. He could have driven off into the sunset in the RV (with unhelpfully diminished vision; hopefully he finds an abandoned Visionworks somewhere) and decided to ride out the apocalypse solo, like Morgan did for a while. Considering his emotional state, it wouldn't be a surprise if Heath decided to go it alone. As long as Heath's death remains unconfirmed, then there's no telling what really happened to him. It's not a satisfying answer, but it does leave room for hope.