The return type of sanitize_term_field() depends on both the $field being sanitised and the $context.

For integer fields

$int_fields = array( 'parent', 'term_id', 'count', 'term_group', 'term_taxonomy_id', 'object_id' );

it returns a non-negative integer. The exact constant value cannot be inferred from the input $value, as the passed $values are strings.

For all other fields, the type depends on the $context:

• 'raw': $value is returned as is, which is handled via the template T; for constant strings, the same constant string is returned.
• 'attribute'|'edit'|'js': $value is escaped--and may be transformed--using esc_attr(), esc_html(), or esc_js(). Assuming the filters in these functions do not alter the type, the returned type is string, even if $value is a constant string.
• All other $context values: $value is not escaped but is filtered, with the filter callback documented as returning mixed. Therefore, the default return type of mixed is retained.