In Odoo.sh, I am currently following the UPGRADE process to migrate from Odoo v13 to v15. Because some of the templates have been removed or renamed in v15, I got 100 errors similar to:

ValueError: External ID not found in the system: website_blog.s_latest_posts

Most of these errors are caused by the third party template on version 13.

How to deal with (Theme-) templates which are not existing anymore in the destination (v15) version but used as inherited template in the current version (v13)?

For example:

website_blog.s_latest_posts.xml is existing in the official src/odoo/addons in v13 but not in v15. And I have custumized this template using inherit:

The "third party theme" module "third_party_theme" inherits from it:

and other of my custom templates which inherit from third_party_theme 's template:

And because the website_blog.s_latest_posts.xml is not existing anymore in v15, during the migration process (odoo.sh), such an error occurs: ValueError: External ID not found in the system: website_blog.s_latest_posts

I have heard about pre-migrate scripts to handle views during migration process: https://www.youtube.com/watch?v=n0jQQlEnUUQ but i can t figure out how to migrate views as described above (pre-migrate.py, post-migrate.py and end-migrate.py) as described in https://github.com/OCA/OpenUpgrade/blob/15.0/openupgrade_scripts/scripts