Minecraft wiki entry for Flint_and_Steel, associated text: In Adventure mode, flint and steel cannot light fires, nor can it ignite nether portals (unless it has the tag CanPlaceOn:obsidian). However, it can still ignite TNT and creepers.
The name "Flint and Steel" is inaccurate considering there is no steel in the game and it's made with iron instead.
Flint and Steel cannot light end portals.