Google has changed my site's page title - Can I opt out of this new update?

There are several quotes from that article that are significant:

"Focus on creating great HTML title tags. Of all the ways we generate titles, content from HTML title tags is still by far the most likely used, more than 80% of the time."

"[In addition to the title tag] we consider the main visual title or headline shown on a page, content that site owners often place within <H1> tags or other header tags, and content that's large and prominent through the use of style treatments. Other text contained in the page might be considered, as might be text within links that point at pages."

"Why not just always use the HTML title tag? For the same reasons we explained when we began going beyond the tag significantly back in 2012. HTML title tags don't always describe a page well. In particular, title tags can sometimes be:

- Very long.

- 'Stuffed' with keywords, because creators mistakenly think adding a bunch of words will increase the chances that a page will rank better.

- Lack title tags entirely or contain repetitive 'boilerplate' language. For instance, home pages might simply be called 'Home'. In other cases, all pages in a site might be called 'Untitled' or simply have the name of the site."

Yes, you can control it: you choose titles that describe accurately the page (even if that does not include your main keyword).