ILR holder acquiring EU/EEA/Swiss nationality

[punktlich02]

X acquired ILR based on his spouse's status (she had ILR at the time, since has been naturalised British).

Subsequently he acquired Swiss nationality (through ancestry and facilitated naturalisation). When Swiss citizens secured EEA rights (most of them anyway) under the 1999 Treaties, he ceased using his TCN passport with ILR and has since travelled using his Swiss passport.

Question: Was his ILR forfeited for non-use (i.e. not declaration upon entry)?

It probably doesn't matter since the only "benefit" he gets is an earned partial state pension and their (adult) children and (minor) grandchildren are already British either by birth prior to 1983 or registration thereafter. But you never know: based upon Swiss nativist political activity and referenda the 1999 Free Movement provision could always be revoked.