In a project I maintain on billing and payments[1], I had a link to Google Ads API. It was documenting how they had hard-limit budget, with notions like "capped actuals" and "monthly with rollover". The explanation was quite good so I keep it around to explain the concept, and why it was perfect for customers to avoid any surprises.
Then the URL[2] 404'd and the API disappeared. I couldn't find any references to a "BillingCap". I wondered why[3].
Now it makes sense: they got rid of budgets as you explained above. Everything's seems to be obfuscated behind a quite opaque Proposals/Deals[4] data structure now.
Thank you, this has been insightful! Sometimes it’s hard to make a report that makes sense. For instance when having to deal with conversion lag I had to deal with this week. https://support.google.com/google-ads/answer/9347141