Not as big as Linux but I know a few FreeBSD shops that run NodeJS apps so it's not entirely crazy to think that there are more and they would want to try Deno. Besides making your OSS software compilable on *BSD/Linux/Mac/Win has historically been a good thing to do anyways.
For a lowlevel runtime (ie V8 itself) I can accept certain lag since there might be some low-level differences in how signals,etc behave.
However for more generic code Linux'isms often signals a certain "works-on-my-machine" mentality that might even hinder cross-distro compatibility, let alone getting things to work on Windows and/or osX development machines.
I guess a Rust binding for V8 is a tad borderline, not necessarily low-level but still an indicator that there's a lack of care for getting things to work on other machines.
It's more than a little surprising that portability between different Unices is not given more emphasis. "Back in the day" a program being portable between Sun Solaris, HP's HP-UX, Linux, FreeBSD was considered a sign of clean code.
Back in the day, Sun Solaris and HP-UX were not end-of-life, and FreeBSD had more equal industry footing with Linux. Now Linux is the clear winner in server OS UNIX by a wide margin. Also, Ryan Dhal worked at Joyent, a Illumos/Solaris shop when he built Node; perhaps that has informed his lack of interest in supporting FreeBSD these days.
Trying to compile it - it's 2.2.0 but better than nothing. I haven't seen any upstream patches for Rust V8 for FBSD so maybe out of tree ones in the ports if it does compile.