https://dev.mysql.com/doc/relnotes/mysql/8.4/en/news-8-4-0.html
https://www.percona.com/blog/mysql-8-4-first-peek/
https://docs.oracle.com/cd/E17952_01/mysql-8.4-relnotes-en/mysql-8.4-relnotes-en.pdf

Please note that some syntax is removed and templates should be also adjusted, like here:
https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/db/mysql_agent/template_db_mysql_agent.yaml?at=release%2F6.4#1223

SQL statements removed.  The following SQL statements have been removed (replacements in brackets): START SLAVE (START REPLICA); STOP SLAVE (STOP REPLICA);SHOW SLAVE STATUS (SHOW REPLICA STATUS); SHOW SLAVE HOSTS (SHOW REPLICAS); RESET SLAVE (RESET REPLICA); CHANGE MASTER TO (CHANGE REPLICATION SOURCE TO); RESET MASTER (RESET BINARY LOGS AND GTIDS); SHOW MASTER STATUS (SHOW BINARY LOG STATUS); PURGE MASTER LOGS (PURGE BINARY LOGS); andSHOW MASTER LOGS (SHOW BINARY LOGS).

The statements just listed have also been removed from all MySQL test programs and files, and elsewhere, where used internally.

Statement options removed.  The following options formerly supported by CHANGE REPLICATION SOURCE TO and START REPLICA have been removed and are no longer accepted by the server. They are listed here for each of these statements, with their replacements in brackets:

• CHANGE REPLICATION SOURCE TO options removed:

MASTER_AUTO_POSITION (SOURCE_AUTO_POSITION), MASTER_HOST (SOURCE_HOST), MASTER_BIND (SOURCE_BIND), MASTER_USER (SOURCE_USER), MASTER_PASSWORD(SOURCE_PASSWORD), MASTER_PORT (SOURCE_PORT), MASTER_CONNECT_RETRY (SOURCE_CONNECT_RETRY), MASTER_RETRY_COUNT (SOURCE_RETRY_COUNT),MASTER_DELAY (SOURCE_DELAY), MASTER_SSL (SOURCE_SSL), MASTER_SSL_CA (SOURCE_SSL_CA), MASTER_SSL_CAPATH (SOURCE_SSL_CAPATH), MASTER_SSL_CIPHER(SOURCE_SSL_CIPHER), MASTER_SSL_CRL (SOURCE_SSL_CRL), MASTER_SSL_CRLPATH (SOURCE_SSL_CRLPATH), MASTER_SSL_KEY (SOURCE_SSL_KEY),MASTER_SSL_VERIFY_SERVER_CERT (SOURCE_SSL_VERIFY_SERVER_CERT), MASTER_TLS_VERSION (SOURCE_TLS_VERSION), MASTER_TLS_CIPHERSUITES(SOURCE_TLS_CIPHERSUITES), MASTER_SSL_CERT (SOURCE_SSL_CERT), MASTER_PUBLIC_KEY_PATH (SOURCE_PUBLIC_KEY_PATH), GET_MASTER_PUBLIC_KEY(GET_SOURCE_PUBLIC_KEY), MASTER_HEARTBEAT_PERIOD (SOURCE_HEARTBEAT_PERIOD), MASTER_COMPRESSION_ALGORITHMS (SOURCE_COMPRESSION_ALGORITHMS),MASTER_ZSTD_COMPRESSION_LEVEL (SOURCE_ZSTD_COMPRESSION_LEVEL), MASTER_LOG_FILE (SOURCE_LOG_FILE), and MASTER_LOG_POS (SOURCE_LOG_POS).

• START REPLICA options removed: MASTER_LOG_FILE (SOURCE_LOG_FILE) and MASTER_LOG_POS (SOURCE_LOG_POS).

Status variables removed.  Also as part of this work, the following system status variables have been removed from the server, and no longer appear in the output of statements such as SHOW STATUS. These variables are listed here, with their replacements in brackets: Com_slave_start (Com_replica_start); Com_slave_stop(Com_replica_stop); Com_show_slave_status (Com_show_replica_status); Com_show_slave_hosts (Com_show_replicas); Com_show_master_status(Com_show_binary_log_status); and Com_change_master (Com_change_replication_source).