In reply to Li-An.

It is 100% possible to use templates for CPTs in the site editor. As of WP 5.9, they just need to be added via the corresponding .html templates. This works the same was as adding the old .php templates.

They are editable via the site editor if included in the theme. So, you could just stick an empty *.html file in there and build from the editor if you prefer.

What's not possible in 5.9 is creating new custom templates directly from the site editor. The dev team limited this feature to a sub-set of templates because they wanted to work on the template-creation flow in 6.0.