The main change seems to be that previously you could turn right off the
northbound offslip and then turn left onto the northbound onslip (should you
so wish), whereas now the traffic turning right from the offslip is forced
into a segregated lane between the two carriageways which takes it past the
onslip without allowing access to it. The only reason I can think of for the
change is that they want to stop traffic from avoiding jams on the flyover
by coming though the junction

This is also why I thought they made the change. Certain several
drivers take this route.

... but since you can drive across into Tunnel
Avenue and do a comfortable leisurely U-turn back to the junction, only
losing a few seconds in the process, the change seems fairly pointless...

Whilst in principle this would be possible. In practice this would be
difficult in the morning peak as Blackwall Lane is so crowded.

However in any case, I believe there are two other ways by which
traffic exiting the northbound offslip can still enter the northbound
offslip.

First is take the middle lane on the northbound offslip (which is
signed ahead-only on the road surface) - you appear to be able to
fairly easily turn right onto the far side of the segregated Blackwall
Lane and you can then turn left onto the northbound onslip.

Second is to take the right-hand lane on the northbound offslip (which
is signed right-turn-only on the road surface) - you end up on the
near side of the segregated Blackwall Lane but for a car, it's just
possible to make a sharp left where the segregation ends and get on to
the northbound onslip (I have seen a car do this).

I
saw some dangerous driver confusion while I was there, and the "No Entry"
text on the tarmac seems like an admission by the engineers that they have
created something baffling and dangerous.

All in all, I think the engineers haven't achieved what they set out
to do. And considered the cost and inconvenience of the works, it
doesn't seem to have been a very well conceived project.

zin92