Hello! Second year War Studies BA here, hopefully I can give a somewhat informed perspective on your question :)

You are certainly correct concerning strategy. Strategy in general tends to be more broadly applicable as it is grounded in several principles, common across all eras. Clausewitz's book "On War", for instance, is still considered a seminal work on the subjects of war and strategy 150 years or so after being published. These common aspects between eras also means that it is easier to make comparisons between different conflicts, even if they are set apart by long periods of time. Terms such as "logistics", "friction", "intelligence", "force ratio"... strategic analysis can be applied across time periods. Analyzing any strategy for its strengths and its failings is a very useful exercise, for academics and for actual strategists.

Operational aspects are also relevant, but become much less so the further and further back you go. I have participated in a module concerning the Second World War, for example, that was grounded entirely in examining the operational dynamics of the war in Europe. Because of the proximity in time, the study of these dynamics is still useful. Understanding the operational dynamics of German "blitzkrieg" operations is still useful, because the tight cooperation between land and air forces in rapid advances is still a cornerstone of modern Western military thought. Obviously, analyzing the operational level aspects of more recent conflicts such as the Iraq War of 2003 and the War in Afghanistan will, due to proximity, consequently be of even more use - at least to a Western historian. But for a modern strategist, being well versed in the operational methods of a Roman legion, or of a Napoleonic army, is not likely to bear much relevance because the way in which wars are fought is so different.

Likewise with tactics. Academically speaking, tactics are rarely studied or considered by themselves. And as I said above, the way wars are fought has changed and will continue to change. The sheer gap in time makes studying the battle tactics of a First World War infantry platoon fairly irrelevant, let alone going as far back as the pre-Modern or Ancient eras. While academically-speaking there is a great depth of study on pre-Gunpowder age warfare, this is primarily done in the interests of history and charting progression, not because it is relevant directly to modern tactics.