As far as I am aware:

Razer now do mainly demos and so picking up damage from combat events aint really ideal if they have a demo the next day and their claw aint working

The Hypnodisc guys I believe took a break and intended on coming back in Series 8 but since such a series never materialised, they didnt

Chaos 2 I think just plain retired (as Mike said). Although George Francis had a robot with him at the 2005 UKRG but I never saw it running (I think it was for tug-of-war or something along those lines) and Chaos 2 was on display there.