I'm not quite sure. Do you mean
a) using sys.dm_os_performance_counters to check deprecated features with a counter has the counter deprecated?
b) I have a function that calls sp_addserver, and that proc call is deprecated?
The release notes, or the page I listed (https://docs.microsoft.com/en-us/sql/database-engine/discontinued-database-engine-functionality-in-sql-server?view=sql-server-ver15) show this, but only a few versions back. MS has changed the way they doc things, so I'm not sure it's as easy as release notes. My guess is that some stuff was just announced, before they build this "discontinued features" page and you'd have to have seen the announcement.
Really, it is a mess. I think XEvents or the perf counter are the best ways in 2019. Since the counter doesn't seem to work in a few instances for me on 2017, I suspect XEvents is best for some versions, but I don't know.
This has a list: https://docs.microsoft.com/en-us/sql/relational-databases/performance-monitor/sql-server-deprecated-features-object?view=sql-server-ver15