(1) also templates should be handled the same way
(2) also hosts should be handled the same way
(3) also triggers should be handled the same way
ZBX-7202 deals with a possibility to leave an action w/o any operations
Just displaying a message may not be the best approach, because when deleting stuff in bulk, it may easily get lost in other messages.
ZBX-7948 asks for this to be documented for now
as for the message, ideally i'd like to see a separate confirmation page or popup, showing which actions are affected by the current deletion 
it's even worse for actions with custom expressions, see ZBX-9943
this seems to be more nasty than i thought initially. it might be a good idea to check your actions and see whether any important ones haven't been silently disabled
there was some interest on IRC in monitoring an important action by ID, and alerting if it suddenly gets disabled. a quick example on how that could be achieved (probably from cron) at https://github.com/whosgonna/Zabbix-Tiny.pm/blob/master/examples/example_check_action_by_id.pl
Sad thing that it does NOT log to audit log too. (host group deletion, used in a condition).
Interesting that an audit message "Action disabled due to deletion of user." will be actually logged.
It happens if you delete an user/usergroup which was only one used in single operation of action.
UPDATE on 2023-10-26, using version 6.4.5.
Now frontend does not allow to delete such a user or a user group and produces error like:
User "udel" is used in "Report problems to Zabbix administrators del" action.
It happens also if the user/group is not the only recipient in the action, which is weird actually is wrong behavior.
Showing something in frontend wouldn't help when using API. I suggest not allowing to delete something that is referenced in action condition.
Just confirming that using version 6.4.5 a host deletion silently disables the action and does not leave any sign in audit log too.
As stated in ZBXNEXT-8982 it would be nice if :
- The deleted condition (host group, host, trigger, etc...) is the last remaining => Message appears indicating that it is still in use and the parent item must be deleted prior.
- There are other remaining conditions => Action remains enabled and a message appears indicating that the condition was used in this or that action.
There is already a reference check in the API:
All that needs to be done is instead of disabling an action return an error.
Thanks to gcalenko for checking it.
Related issue: ZBX-23760