I actually wrote an article on upgrading to 8.3 earlier this month -
https://www.bigredseo.com/upgrade-php-for-faster-wordpress/
There have been a few things depreciated, but it's mainly on calculations.
One BIG change that we noticed was on the String to Number Comparisons, the non-strict comparisons. We do a lot of this when it comes to custom code. Usually you're using "==" (non-strict) and "===" (strict). So you can think of "==" as a partial match.
In PHP 7.x this statement returned TRUE, but now they return FALSE.
Code:
0 == "foo"
0 == ""
42 =="42foo"
create_function() has been completely removed. No longer needed.
match is now a reserved keyword - so if you have created any functions, beware.
the default error_reporting level is now ALL - everything. Be sure you check your php.ini and redefine to exclude NOTICE and DEPRECIATD if you don't want to see those in your error logs. There's actually a bunch of changes that have happened with the error reporting where things that used to throw errors not just throw warnings or notices. So logging can be a little more detailed when needed.
mktime() will get a UNIX timestamp, however, you must now call with an at minimum the hour argument, and optionally call minute, month, day etc. So instead of mktime() you'll swap to using time().
So there's a few changes, but for most people, they're not going to notice much. Plugin developers etc need to be on the watch for the updates.
We've updated several WordPress sites without issues, but we have a few with custom code that had to be rewritten to handle the new functions.