Custom playbooks in Madden 20 remain unchanged. However, as you've likely heard, there are many additions and omissions regarding the actual plays themselves. I built a quick 60 play book and I didn't have any problems with audibles. The formations stacked nicely, since I always start with a book with formations I am going to keep and make sure I leave at least one play up per-those formations before deleting those that I don't want. We still cannot slot plays in our book where we would like them to show up, which is somewhat frustrating for me.
The big change I noticed is gameplanning. We can now select up to 30 plays per-situation and they will show up under suggestions. We only could do this with 9 plays last year and they showed up under frequent playcall window.
Keep in mind, if you do not suggest 30, you will see plays repeated. Furthermore, you will be able to select from the newly added pass only or run only playcall windows if you wish. This will allow you to find your play more quickly.
I sent Beta feedback about the need to improve this further, as I would like to see the gameplanned plays "stack" according to star rating awarded, as was the case back in Madden 12.
They are at least headed in the right direction and they did make some improvements. They appear to have the gameplan playcall template at least in place this year. Overall, still not as organized as I'd like to see it.