I think it depends only on what you want search engines to find on your site. If you want them to be able to find every word in every corner, do not use ajax.
If this is a private area in which the user must log in, there are no restrictions. The search engine cannot find such content.
For web applications, I started doing the whole page in javascript and ajax. Therefore, you will never have to reload the page. It makes your life easier.
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