As Bitwarden states in the core readme (https://github.com/bitwarden/core#requirements) "These dependencies are free to use."
And if you lookup the licensing on MSFTs website (https://www.microsoft.com/en-us/sql-server/sql-server-2017-p...) you can see that the SQL Server Express version is free.
Note: No, I'm not an MSFT employee and not related in any way to the mentioned company. I'm just a (not so regular) human being who first looks up information before posting statements.
The README requires SQL Server so I might be excused thinking that it would work only with that software. Nice to know that it does work on the free version too. I googled and looked on MS site and didn't find out if SQL Server Express exists for Linux too, but I'm on my phone right now. I only managed to involuntarily download a .exe :-)
There you go: