Prevent trash/delete action on specific post types

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Here’s another approach using the map_meta_cap filter that’s applied within the map_meta_cap() function within the has_cap() method of the WP_User class (PHP 5.4+):

add_filter( 'map_meta_cap', function ( $caps, $cap, $user_id, $args )
{
    // Nothing to do
    if( 'delete_post' !== $cap || empty( $args[0] ) )
        return $caps;

    // Target the payment and transaction post types
    if( in_array( get_post_type( $args[0] ), [ 'payment', 'transaction' ], true ) )
        $caps[] = 'do_not_allow';       

    return $caps;    
}, 10, 4 );

where we target the delete_post meta capability and the payment and transaction custom post types.

As far as I understand and skimming through the get_post_type_capabilities() function we don’t need the map_meta_cap argument set as true, in the register_post_type settings, to target the delete_post meta capability.

ps:
Here are some good descriptions of the map_meta_cap filter and helpful examples by Justin Tadlock here and Toscho here. Here I found an old example that I had forgot I wrote, that we could also adjust to avoid trash/delete on some given pages and user roles. Here’s an answer by TheDeadMedic that links to an answer by Seamus Leahy regarding register_post_type approach.
Here are some more examples on this site. Hope it helps!

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