What regulation addresses whether you need to sign in or out of leave physically after IPPS-A took effect?

You're confusing policy with regulation.

The regulation requires that you are physically present at your post, duty location, or home (where you normally commute to work from) in order to start/end a pass or leave, but there is no regulation that say you must physically sign out on leave.

Many commanders have a policy requiring you to physically sign in/out so someone doesn't try to game the system and get in trouble (especially for junior Soldiers) because something unexpected happened. However, like all local policy it will depend on your unit, which installation you're on, etc.

AR 600-8-10 will be the Regs you want to reference, however...since this current edition of the reg is 3 years old, and published before IPPS-A came into creation, I would err on the side of caution and recommend the following:

1) Print off a copy of the IPPS-A Leave (Absence) Request that is signed approved and provide that copy to Staff Duty. This will still accomplish accountability for both Company and BN,
2) Check with your Company Command area to see if/what policy they have in place for this type of situation.
3) Besides checking with Company, see if your BN has a policy regarding this situation.

SGT (Join to see)
You might try Army Regulation 600-8-10 Leaves and Passes.

https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN30018-AR_600-8-10-000-WEB-1.pdf