(This is a bump, with an improved example, of my earlier note on the subject.)

In MKIV, \preventmode does not work. It appears to do the opposite of what is intended, and enable the specified mode. It also does not prevent the explicit enablement of a mode (as can be seen by uncommenting the commented line in the MWE).

I have not tested it with MKII , where it might work. (Will ConTeXt online ever be restored?)

This goes back to TL14 and likely earlier.

Do I misunderstand what \preventmode should do, or have I misused it?

And why are there {curlies} instead of [squares] around the mode name in \doifmode and friends?

MWE, run context MWE.tex:

\starttext
\preventmode[X]
%\enablemode[X]
\doifmode{X}{!}
\stoptext

--
Rik Kabel