SQL Server 2017 Availability Groups - Can a Long-running transactions on a primary replica block a secondary replica from seeing updates?
Can a Long-running transactions on a primary replica block a secondary replica from seeing updates?


Only if it blocks the REDO thread(s) which would require the use of DDL on the primary and a workload that is blocking said DDL from acquiring the needed Sch-M lock on the same object(s).

Long running transactions don't "stop" you from seeing updates but a plethora of other items will, such as:


Slow Redo
Slow network
Slow disk
CPU/Memory pressure
Synchronous/Asynchronous
Readable secondary replicas use SNAPSHOT isolation under the covers



  Has SQL Server 2017 (or 2014/2016) removed this limitation?


AFAIK there was never a limitation except that your log is going to grow which could cause other issues. It could also present issues with readable secondary replicas if this impacts or blocks ghost cleanup - that's generally what I've witnessed and in those cases the transactions were open on an extremely busy system for 24-48 hours. Since Snapshot isolation is used, your tempdb is going to take a hit due to version store and there is a limit. I'm not exactly sure what the quoted text you have is referring to as it doesn't explain the scenario(s).