i want to send email to user when the custom taxonomy is changed

You can use the pre_post_update hook to get the new value before the post is saved. This way you can check if the value has changed (old value is still in the database at this point) and do your magic.

See https://developer.wordpress.org/reference/hooks/pre_post_update/

Related Posts:

  1. How to get all taxonomies of a post type?
  2. Permalinks: custom post type -> custom taxonomy -> post
  3. How to Add Tags to Custom Post Type?
  4. Mixing custom post type and taxonomy rewrite structures?
  5. Can multiple custom post types share a custom taxonomy?
  6. How to Add Custom Fields to a Custom Post Type?
  7. Efficient Taxonomy Intersection
  8. Display current taxonomy term when inside custom post type
  9. limit selection of custom taxonomies to one?
  10. Set relationship between custom_post_types
  11. $wp_query->queried_object->ID throws warning: Undefined property
  12. How to list custom taxonomy categories?
  13. clients list using wordpress
  14. Custom taxonomy query for a custom post type
  15. Extend walker class with custom post types [closed]
  16. How to do a custom bookmarks post type?
  17. Custom permalinks – post type – hierarchical taxonomy’s
  18. Taxonomy posts on Archive page
  19. Taxonomy list. Order by a specific custom post type count
  20. Rewrite CPT slug with two taxonomy slug instead of one
  21. Archive or taxonomy pages not working for custom post type
  22. Custom Taxonomy returns no posts
  23. How to display Custom Taxonomy of Custom post type
  24. Custom content type meta_box_cb does not run
  25. associate custom post type with tags with specific pages
  26. Building an Advanced Search (text, tags, category, custom fields) – Getting the wrong SQL query
  27. Custom Taxonomy breaking pages permalinks
  28. Custom post types and permalinks
  29. taxonomy template for custom post type the same as the archive
  30. Get categories and descriptions from custom post type
  31. Problem with custom post types, taxonomy and permalinks
  32. How to create new category for custom post type?
  33. Fetch taxonomies by custom post type id array
  34. Repeating posts when paginating random posts
  35. Normal pages as children of my Custom Post Type
  36. Query/list all terms and their custom post count
  37. Remove url rewrites for registered taxonomies
  38. Received nothing after executing AJAX post function
  39. How can I auto-assign a CPT post to category in a custom taxonomy when published?
  40. How can I make a custom post type sticky?
  41. Mapping Subdomains in WordPress to give the appearance of a localised site, best approach?
  42. Get Taxonomy Term Title by it’s URL
  43. WP_Query inside foreach loop returning same value for all options when filtered using ajax
  44. Group CPT posts by custom taxonomy
  45. Limit number of custom posts per taxonomy
  46. Number of Custom Post Types published are not being shown in the custom page
  47. Metabox not show in categories custom post type cmb2
  48. Categorize custom post type
  49. Load Custom_post_type categories post with ajax
  50. Custom Permalink Structure for Custom Post Type, Custom Taxonomy, Under a page
  51. WP-API select custom taxonomy when creating a post
  52. List of Posts and Categories
  53. Why is my WP_Query outputting my entries twice?
  54. How to display Related Posts based on number of taxonomy terms matched
  55. How to solve this custom post type goes to 404 page?
  56. Get term name without a foreach loop
  57. How to Set Taxonomy Object Description?
  58. Custom Post Type with Custom Taxonomy as Category
  59. WP_Query() with custom post type and taxonomy — get all terms?
  60. Add term list as class
  61. Taxonomy archive 404ing (not term archive)
  62. Variable not working in WP_Query
  63. How do I get the custom category URL from the custom post type?
  64. Alphabetically sort a taxonomy.php template by post title
  65. Show Taxonomies with admin area for custom post type?
  66. get_terms() parent, child and grandchild
  67. Custom Post Type Custom Archive Page Not Working
  68. Enabling permalinks disables custom page template
  69. Getting custom taxonomy posts on archive page
  70. Multi Parameter search with custom post type category and tag
  71. How to create Custom Taxonomy Search Drop Down for specific Post Type
  72. Can I set the post_type query_var as a link is clicked?
  73. Remove cpt slug from url and use custom taxonomy instead breaks all other cpt
  74. How to display custom taxonomy term specific post?
  75. Get taxonomy name used in a current post
  76. Changing permalink of default Post with custom post
  77. get_queried_object not work in taxonomy page
  78. wp_insert_post deleting previous post custom meta
  79. Custom Taxonomy Archive generates 404
  80. How to create a permalink structure with custom taxonomies and custom post types
  81. Support for author does not show author column in post table in dashboard
  82. How to get all posts related to a taxonomy?
  83. Custom post types not displaying per category
  84. Create 2-layered dropdown menus for custom taxonomy and custom post type
  85. Force WordPress to only match URL in category
  86. Hierarchical taxonomy in permalink structure
  87. Can I create a taxonomy term that mirrors the name of a new custom post?
  88. Rewrite for custom type and custom taxonomy
  89. How to create TEMPLATES for TAXONOMIES? What I have to change in this code?
  90. Custom Post Status & Taxonomies
  91. Why won’t this rewrite rule work?
  92. Archive.php is not displaying tags
  93. Show Custom Post Type taxonomy term that matches custom field
  94. How to not have the items of the subtaxonomy within the parent taxonomy
  95. How to save an integer as taxonomy term?
  96. Sort custom post type items by taxonomy description
  97. How to Display Custom Taxonomies in an Un-ordered HTML List
  98. how do I create a breadcrumb to work with multiple custom taxonomies
  99. Get custom post type parent category
  100. How can i change url structure of cpt like this?
Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)