In the last few days 3 friends have tested positive just a day after separate flights from UK back to USA.
It is said that planes have excellent aor filtration systems, but it's not perfect. Over hours you can breath in infected air from your neighbour before it gets suckled out. And then there's proximity during boarding and using the toilet.
Don't rely on built in systems. Wear an N95.
Sure it's an anecdote & there's no proof infection was acquired on the flight, but you can't be too careful!