Voice syllabus feedback
There are no proper vocal warm-ups. In 'workout' there are some vocal health exercises and some body warm ups but no actual vocal warm ups. The first thing that should have been put in the app is a 10 minute warm up routine of lip trills and scales.
There is nothing to train people's registers to get people to coordinate using chest voice, falsetto and head voice and learning how to blend them smoothly without big gear changes
There's nothing on mix voice
There's nothing on vibrato
There's nothing on agility and developing runs and embellishments
There are no maintenance exercise routines for vocal fitness
There's nothing for an advanced singer. Advanced singers want something like this app to act as a gym, a place they can go to reinforce good habits and have a bunch of exercise routines and challenges that can test and train specific facets of the voice, eg breathing, mix voice/connection, agility, endurance, stability, resonance etc
If you add an extra 5 levels to the syllabus, you can teach these critical skills and then place specific workout routines for those skills in the workout section.

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Unanonymous commented
Please file this as feedback directly to the company as well.
There used to be warmups and such for singers on guided lessons, but they were an unpopular feature and got removed. After all they need to give the market what it wants or they have no business. They may still there (on some videos maybe?), just not mandated or actively encouraged. Dunno.
I think there's a lot of room for growth all across Ys' spectrum. They'd need to tailor the ue differently for serious learners and people looking for fun while learning on the side. That would need a lot of effort, I hope they'll be able to pull it off some day. They could at least have an option for those warm ups.