I have noticed this as well. @adam was this feature removed in 5.0? All documentation still shows the availability and screenshots for it, but the last documented reference I can find to the designer feature is from 4.1.2 on your YouTube series. There’s a comment on the video complaining about this very thing as well.
Manually visiting /apps/Designer/<app name> returns a 404, as shown here:
The designer was removed in v5. There is actually a depreciation warning in v4. As for the screenshots and docs, I’ll have to do a bit of clean up there. If you have specific links, please pass them along.
I thought there were more but it seems like my brain was tricking me - I think the main documentation out-of-date issues are in the cmdlet references which I have seen getting updates over time anyways. The only real reference I can find, which would spin some folks out expecting this UI, would be in the main Pages documentation.