I spoke with Odoo support today, and this has been completely removed. In their response: "You won't be able to customize the PDF reports like normal. A ticket will have to be made for the development team to make the customizations".

This is absolutely ridiculous. I love the Odoo system, however downgrading users like this is insane. This now needs to be a custom development which costs both money and time, when I could easily do this with previous versions of 16.3.

Doing a custom development requires a big long written request, then they come back with their explanation of how it will be done. Then I pay them to make the customization at their rates.

Most PDF customizations in the past (pre 16.4) were simple and easy to do and took hardly any time at all. Every other CRM and ERP system I've dealt with has a PDF template editor. Odoo used to, and now they've removed it. And it has angered me greatly.

I am missing the "Add" tab as well. 

Adding "/" to the editor does not really help as I then only can select certain fields, but I cannot change the layout, add blocks, etc. Anyone any ideas? 

Update: Talked to odoo support and they confirmed that the "add" tab was removed and everything needs to be done with "/" directly in the editor. This is unfortunately a down-grading feature..... 

No solution yet, you can modify the document by coding, but they said that it´s not going to be updated.