"get_{$adjacent}_post_where" allows one to directly alter the SQL WHERE clause, so it is extremely powerful in its ability to customize the query to find the next/previous post. You can alter this clause to exclude posts, include posts, and do whatever else one does with SQL queries. This includes using various columns of various tables to qualify results, including title, slug, date, etc. Since all these criteria work off the same filter, I wouldn’t say any one is easier or more difficult, just whatever is most convenient would be the one to use.
Besides the post where filter and the previous excluded terms filter, there is another filter for the JOIN clause, so you could link in other tables besides those already used to construct quite elaborate criteria. This is no doubt beyond your personal ability, my intent is merely to merely impress upon you that there are few limitations regarding the alteration of this function to someone knowledgeable enough.
Using a particular category to exclude certain posts is actually a good idea because it’ll be easier to exclude other posts in the future. Merely assign the exclusion category. If we were to stick with the post where filter, any future post that needs exclusion would need to be added to the custom PHP code, a much more daunting task than assigning a category!
The catch is that excluding the category from the next/previous post template tags still requires hooking a couple filters and altering the passed value. There is another possibility with this approach though. It entails creating your own custom template tags that use get_adjacent_post() just like the core template tags (next_post_link() etc.), except they make use of the excluded terms parameter of the function itself, instead of relying on the filter.
This approach may be desirable because most of the custom code is copy/paste, just renaming the functions and using that one parameter. But honestly, using the excluded terms filter is simpler once you understand the concept. Understanding filters is difficult to grasp at first. At some point the concept sinks in and you’ll think “Oh, I see! It’s so simple!” Filters (and actions) are integral to customizing many aspects of WordPress, so it’s worth the time to study these in detail until they are fully understood.
Aside from constructing a filter callback function and adding it to two filter hooks, the only thing the callback really needs to do is this: $passed_array[] = $term_id;
More on filters and actions can be found here: https://developer.wordpress.org/plugins/hooks/
The reference is for plugins, but filter hooks can be right on the template if that is the only time the hook is required. Hooks can also be on functions.php of a child theme. Child themes are strongly recommended any time you customize a theme template (or anything else about the theme) so your changes are protected should the theme be updated.
