Frontlining is a common role in lower TTK games like LoL and Overwatch, but I haven't been able to find a well defined description of what it actually is and it probably varies a bit from game to game. To me, a frontline character is one that control space with forward positioning, and are typically high durability, tank characters who aren't afraid to take a couple punches to force enemies to take a step back to let their team step forward. I would think of characters like Reinhardt from OW or Sion from LoL as some good examples.
I haven't felt that I've really been able to identify this type of gameplay in Supervive tho. The low TTK means that everyone is afraid of taking punches, so the kind of forward positioning that I'm think of, the kind that puts the "front" in frontline, means you'd get chunked by free LMBs before you accomplish anything, and you don't really need anyone to tank things in more neutral positions, because you can WASD to dodge long range abilities reliably enough.
I'm sure Oath probably fits my interpretation of a frontliner to some extent, but I don't understand how to play/position on him and I've hard inted the few times I've tried him. It doesn't really feel like I'm able to frontline with him in the sense that I think of frontlining, but that could easily just be a skill issue on my part. He just doesn't seem to make sense with how I think about or approach the game, so he's super unintuitive for me.
The next closest character that I feel like I do understand would be Brall, but the way in which I feel Brall enables his team to take and control space is by diving or threatening dives. "Diving" feels like it works because it forces actual resources out of the enemy, and Brall does this well because he has a lot of control over how hard he wants to commit when he jumps in. If I'm playing for space I'll jump in and "fake" a commit to try and pull enemy attention and trade CDs, allowing my team to take space and if I didn't int, be on fairly equal footing once I disengage.
I'm sure y'all have different interpretations of what "frontlining" means, and different ways of thinking about the game, which I'd be curious to hear about 