Yes & no.
If you take improvisation in the strictest sense of the word then none of Adrian's leads are improvised in the studio; since they are (if this is accepted as fact) fully worked out, before they are recorded. I'd imagine almost every metal guitarist does this. Hardly any lead work sounds like it was recorded entirely spontaneously. However, in a looser sense, they are still improvised guitar leads over a rhythm section; they still involve the free development of notes in whatever key the rhythm part is in, built around scales & modes --anything goes, but with rules (i.e. in key). In this sense all of Maiden's lead/solo work can still, in my opinion, be fairly referred to as "improvised".
According to early interviews (i.e. for NotB, etc) Dave didn't have leads worked out at all & just played those off the cuff. This is true improvisation. However, once cut to vinyl, these still became, evidently, fixed entities; as Dave still plays many of them pretty close (to the studio recording) live. Janick's also don't sound that polished. Adrian's sound very worked out. I don't know if playing their leads live, as they were recorded, is something they feel that fans want to hear, or just something that's easier. But it's pretty clear these strictures (at times) are something both Dave & Janick yearn to break free from. I don't see much evidence that Adrian yearns to lay down some live improvisations. Not a criticism, just an observation.
I also think all three guitarists (& Steve) just have a totally different view of what leads are (from each other & from fans); what they should sound like; & how important they are. In Shirley's diaries it described how Steve didn't even feel he needed to be in the studio for the recording of the lead work; Dave (as discussed) probably doesn't (fully) work leads out, before recording; Janick probably doesn't pre-plan them either; live, Janick "butchers" Adrian's leads (i.e. plays something different); & live, Dave is "lazy" (i.e. plays something different). The expectation that these should sound: exactly like the did on the album & that albums leads should be fully worked out musical masterpieces --are expectations that are never going to be met. Not because anyone is incapable or has lost something (or some such rubbish); but because (besides Adrian) nobody is trying/aiming to do this.