SQL Server Management Studio - Why is Dark Theme Disabled by Default?
Because it is a beta feature according to this blog, and this suggestion as well as this one for 18.1.
As a workaround, you can get some custom themes from the web. A favorite of mine is Aaron Bertrand's. He explains how to do it manually (so you can roll your own) and also provides a file you can just upload. Other blogs mention this as well.

Basically I'm curious if there is some sort of consequence I'm unaware
of by turning this feature back on.

Like all BETA features, it isn't supported and thus if it breaks something else that's on you.