I've just tried but cannot add a Trailing stop to a "Limit" order. I'm on the standard IG platform, having joined last month (so presumably the "new" platform), compared to 2018.
I found a forum discussion from 2018 discussing this; the IG reply was "a coding and logic issue" and that it was on the list on things to do for the technical developers.
I wonder if there has been any progress with this please?