Using a modified version of the YouTube app to access Premium features without payment is a clear . As the terms state: "You shall not download any Content unless you see a 'download' or similar link displayed by YouTube". By agreeing to the terms of service, you are contractually bound to this rule, and by breaking it, Google could potentially seek to recover lost income, including loss of advertising revenue.
If you are a casual user, the risks likely outweigh the benefits. For those who watch YouTube frequently, paying for the official YouTube Premium ensures a stable, safe, and fully supported experience. youtube premium.ipa
Free Apple IDs require you to "refresh" the app every 7 days to keep it working. Using a modified version of the YouTube app