Thanks for your topic.
Without knowing the exact thread , I can understand that in a long term thread, the thread can go off topic a bit at times . Often even if it is not exactly what the topic title says,it may be discussing issues that are related to it even if in a general way.
Also some people find a long term thread and are new and lost so they need support before maybe suggesting another thread.
Sometimes the original person who started the thread has left the forum. Sometimes the person who started the thread has changed the way the thread was first intended and maybe feels it has evolved with time.
If you didn't want to actually say get back on topic, you could write a post that is back on topic and end with a question and hope people will follow your example.