The data stored in the cache should be raw database query results, not
`WP_Term` objects (which may be modified by plugins, and may contain additional
properties that shouldn't be cached).
If term relationships caches were handled in `wp_get_object_terms()` - where
a database query takes place - it would be straightforward to cache raw data.
See #34239. Since, in fact, `get_the_terms()` caches the value it gets from
`wp_get_object_terms()`, we need a technique that allows us to get raw data
from a `WP_Term` object. Mirroring `WP_User`, we introduce a `data` property
on term objects, which `get_the_terms()` uses to fetch cacheable term info.
Fixes#34262.
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`get_term()` now returns a `WP_Term` object, instead of a `stdClass` object.
Cache support and sanitization filters for individual terms are now more
centralized. For example, `get_term_by()` is able to cast results of its query
to a `WP_Term` object by passing it through `get_term()`.
The `$taxonomy` parameter for `get_term()` is now optional, as terms ought to
be unique to a taxonomy (ie, shared terms no longer exist). In cases where
`get_term()` detects that the term matching the specified term_id is from the
wrong taxonomy, it checks to see if you've requested a shared term, and if so,
it splits the term. This is used only for fallback purposes.
The elimination of shared terms allows the caching strategy for terms to be
simplified. Individual terms are now cached in a single 'terms' bucket.
Props flixos90, boonebgorges, scribu, dipesh.kakadiya.
See #14162.
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