As @grb said, “Not a big deal”. The username is not intended to be a “secret”, so it does not really affect the integrity of your vault.
It is more about avoiding an annoyance. If someone knows your username, and tries to login to your web vault, you get an annoying email. Presuming your email provider supports it, setting your Bitwarden email to a plussed address ([email protected]) is the least-impactful way to cut down on these.
Don’t go changing Bitwarden to an obscure email that you never check, though. It is important that you receive and pay attention to emails from Bitwarden so that you can remain vigilant about things such as unexpected successful logins.
Your email address can be changed on the web vault. If you do decide to change it, I would first create an export (backup) just in case something goes wrong.
Not today, but it is currently the highest-voted feature request. It has the attention of Bitwarden, but has not yet made it to the roadmap. Feel free to add your vote (well, 1 of 20) to the feature request. Maybe we can push it into 4 digits.