Any updates on this one?)

Nope. Happy to take concrete suggestions for how to improve the UI :)

At least provide a legend what these letter mean and do?

Y: yes
n: no
v: ?
d: ?
a: all?
q: quit?
j: ?
k: ?

If you press ? it shows a legend:

Shall I apply this migration? [Ynvdaqjk?]: ?

y: apply this migration
n: don't apply it

v: view this migration in full

d: apply the selected migrations, skipping any remaining
a: apply all the remaining migrations
q: cancel without making any changes

j: skip to next migration
k: back up to previous migration

?: show this help

That's really not discoverable enough though.

To improve it, I propose removing some options to make the list less overwhelming (I don't think d, j, k are useful) and then explaining how to access the legend:

Shall I apply this migration? [Ynvaq, or press ? for help]: