Is it still possible for fighters with a martial arts background and fighting experience to move into amateur boxing? or is this no longer a possibility?

It is, in fact more possible than it used to be.

Do the right thing and declare your activity in martial arts and most areas are fine with that these days.

It's a slight contradiction in that the ABA don't recognise Thai & Kickboxing and usually dismiss your experience when matching, but it is relevent if you are Boxing in novice class. Open class is not so relevent.
Also, I think the main reason for declaration is to ensure that you don't come to box straight off a stoppage in another sport if a medical suspension should be in place.

Further to a comment made recently, there is no rule in place officialy to stop a Pro in martial arts competing as an amateur in Boxing - mainly because the ABA do not recognise our sports pro or am!

anything is possible ,people are changing the rules all the time on wot is allowed and wot isnt . wot we couldnt do last year is differant to wot we can do this year . they wont like it but they cant stop you .

u can now go on courses in boxing even tho u dont belong to a boxing gym proving things within the abae r moving forward