Well, see attached.
Interestingly, it does with \mainlanguage[de]. But, not for \language[de]

\enabletrackers[hyphenation.applied.visualize]
\startexceptions[de]
	au{f-}{f}{ff}as-sen
\stopexceptions
\mainlanguage[de]
\starttext
auffassen
\language[en]
auffassen
\language[de]
auffassen
\stoptext

> -----Ursprüngliche Nachricht-----
> Von: Hans Hagen <j.hagen@xs4all.nl>
> Gesendet: Montag, 22. März 2021 12:48
> An: Maier, Denis Christian (UB) <denis.maier@ub.unibe.ch>
> Cc: ntg-context@ntg.nl
> Betreff: Re: AW: [NTG-context] Questions regarding \startexceptions and
> ligature prevention
> 
> On 3/22/2021 12:11 PM, denis.maier@ub.unibe.ch wrote:
> 
> > Besides, is it intentional that this here will suppress the ff ligature?
> >
> > ---------------------------------------------------------------
> > \enabletrackers[hyphenation.applied.visualize]
> > \startexceptions[de]
> > 	au{f-}{f}{ff}as-sen
> > \stopexceptions
> > \mainlanguage[de]
> >
> > \starttext
> >
> > auffassen
> > \stoptext
> > ---------------------------------------------------------------
> >
> > I don't see why that should happen here.
> afiks it doesn't suppress
> 
> 
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