Joomla 3.5 Why doesn't include head work after update?

Question

CMS

I updated Joomla from 2.5 to 3.5 on the site - the system itself was updated without jambs (for the 20th time after disabling absolutely all modules, components and plugins)
But the template fell apart. Well, it’s understandable, you need to file it under the new version, I thought and got into Google. I read how to write templates, etc., but did not understand why it is not processed by the kernel.
I copied and pasted the template head from the Habr tutorial - to no avail. Copy-pasted from a working default template - also to no avail. As a result, Fig. 1

It was clear to me that error.php was being processed, since on a broken template it also displayed 404 in addition. I replaced the head in both the index and the error, but Fig. 1 did not want to change, although the source code of Fig. 2
Why Joomla doesn’t process head is a mystery to me, how the front-end generally crashes when the back-end is updated, and why, under similar circumstances, this system is used - it’s the same mystery to me, but since they use it, I thought, it means someone has come across and who -something knows, and decided to write to the toaster lol