I don't think its ever been automation's responsibility to keep an aircraft OCTA. Obviously, VNAV is capable of flying and meeting the level restrictions and forming/flying an approach path, but I believe it has always been the pilot's responsibility where if they haven't been cleared to enter and leave airspace ( or in this case via the RNP approach), to maintain separation from airspace.
I think in the case of this approach. Some aircraft ( potentially heavier, unfamiliar to the local environment aircraft such as private jets or potentially larger aircraft) it isn't efficient or safe to be descending early on that initial leg or the approach as whilst it does get you below the CTA step, it places you smack bang in the middle of the training area in particular with aircraft headed to the inbound reporting point TWRN. Therfore, it would be expected that in some scenarios, aircraft would be cleared to enter CTA via the RNP approach into BK and thus can fly the aircraft as depicted on the chart and negate CTA steps.
If Garmin put an altitude restriction to get the aircraft OCTA by the step, aircraft that had been cleared to enter CTA would now descend OCTA if following that VNAV profile.
I think at the end of the day, id imagine it wouldn't be feasible for there to be an option to select if the pilot had been cleared to enter CTA or remain OCTA, so therfore build the approach and code it as any other approach, and make the pilot responsible for remaining OCTA as they should .
Practically, you would brief that you must descend from at or above 3700 to 2500 by 6.5nm WI. This essentially makes the RAKSO- WI leg 6.8nm ( as you need to be at 2500ft by 6.5nm WI) rather than the 13.3nm unrestricted leg.
6.8nm x 318ft ( 318ft per nm for a 3 degree descent path) = 2162ft. This means that to fly a 3 degree descent and be established at 2500ft by the step, you can cross RAKSO at 4662ft ( 2500ft+2162ft), rather than the published 3700ft. This enables you to stay higher longer and delay your initial top of descent from cruise by a few miles.
If you indeed crossed RAKSO at 3700ft, you could level off and maintain 3700ft. 3700ft-2500ft= 1200ft to lose. 1200/318ft= 3.77nm. Therfore, your descent should begin at 10.27nm WI to fly a 3 degree descent from 3700ft to 2500ft and not clip the step at 6.5nm WI
Obviously, you could also cross RAKSO at 3700ft and descent straight away to 2500ft. Whilst this is less complicated, you are flying longer at low altitude which is less efficient and also having more conflict with training area traffic.