This plugin uses https://developer.wordpress.org/reference/functions/rest_url/ to get the rest url. I do not see any deprecation note in that function. Also i do not see any new api to get rest url. WordPress core also using this function. Can you ask that plugin developer what is the new api for this?
If you want exact use location (or filter to change url) you can see
For sending message: https://github.com/shamim2883/front-end-pm/blob/04f0cf3ada1e32a6d6bb5982af59212492d6a3ec/functions.php#L246
For getting user list: https://github.com/shamim2883/front-end-pm/blob/04f0cf3ada1e32a6d6bb5982af59212492d6a3ec/functions.php#L322