Since F#10 a nowarn directive without warning number throws an error:

let one = 1
#nowarn // interrupt compilation
let two = 2

now fails with error FS3875: Warn directives must have warning number(s) as argument(s)

I used a lot that to force interrupt compilation, for instance to avoid a static member extensions being compiled as an intrinsic, stuff like that.

Having said that, I reckon the impact of this regression is not huge and the severity is not that bad as it can be easily workarounded by adding a warning number, for instance itself 3873.

However I am reporting it for the record, and maybe it should go to a list of known regressions for F# 10