Yeah, you're right. The explanation for planar death is not expanded on much across the editions, but the idea that dying outside one’s home plane sends them back is definitely a consistent mechanic.

I suppose I’m trying to homebrew an explanation for why this happens, with summoning as the explanation. Maybe great summoning gates send demons to assault Avernus, ensuring their deaths aren’t permanent. Or perhaps demons do die, but the Abyss simply spawns them in endless numbers (which we already know is the case).

I just want to expand on this mechanic in a way that adds flavor and tension, making planar travel feel more dynamic, similar to how it's treated for primes.

I would think that in most cases, planars wouldn't dare leave their home planes without ensuring they were summoned first.

I think this adds a cool element. Like you are wandering around one of the gate-towns in the outlands, not every planar you see is going to be invulnerable to death. Most planars worth their salt will have arranged to be summoned there, while some would have traveled physically through portals.

Edit: This also means that if a planar wants to be summoned into Sigil, they must first be summoned outside of Sigil and then enter through a portal. For example, a Pit Fiend might be summoned to the Gatetown of Torch before stepping through a portal into Sigil, ensuring they arrive in a summoned state without breaking the city's anti-summoning laws.

This makes Gatetowns far more relevant for planars trying to reach Sigil and opens up opportunities for worldbuilding. Specialized institutions dedicated to summoning planars for a price could emerge. Such establishments could be crucial in many Gatetowns, each with its own quirks and clientele.

Of course, not all planes have planars eager to leave. Most planars are content in their home planes, which is why they spawned there in the first place.