It depends on what you're doing. I've been in Elixir web dev for about 7 years and never really needed to write or understand Erlang, outside of my work on Hologram. That said, Erlang is always there under the surface - it "leaks" into Elixir in places if you look at the stdlib, and understanding it gives you a better mental model of the whole platform. I'd say you don't need it on a daily basis for most Elixir work, but the runtime and OTP are incredible pieces of engineering, and getting to know Erlang better will only make you stronger in this ecosystem.
Just wanted to echo that my experience is almost identical (production elixir/phoenix for 10 years), and also very rarely touch erlang. But it does leak in every now and then.
And it'll feel less intimidating once you code in elixir, but I still find it more cryptic and less approachable the 3-4 times I've had to splunk into erlang to debug something in the last decade.
