Starships: Battlecruiser
In real life, battlecruisers have always been a controversial and contentious issue, but I thought I'd approach this from a more statistical angle.
While the recognized features of the battlecruiser, firepower and speed at the expense of armour, could be universally and timelessly applied, it took Fisher to bring it to the fore.
The sweet spot appears to be twenty sevenish thousand tonnes, speed depending on (sub)tech level, and armament nine twelve inchers, eight to ten twelve inchers, eight thirteen to fourteen inchers, or six fifteen inchers. No larger "pure" battlecruiser was completed, some were converted to aircraft carriers
This translates in Traveller terms to hundred thousand tonnes, if you assume that at tech level twelve weight was at parity with battleships, but that most naval staffs started preferring to invest in more survivability, rather than speed and range, at tech level thirteen. The Solomani constructed the Zeuses at one fifty kay, while the Imperium designed the tech level fourteen Diaspora to weigh in the same as a battleship, at two hundred kay. Both were singular classes, though for different reasons. For the Solomani they were an intermediate step, while for the Imperium filled a niche requirement they felt they didn't need or actually want.
While battlecruisers are the perfect peacetime capital ship, providing a fast response to potentially critical situations, and power projection in low risk scenarios, arguably useful for short, sharp conflicts. Their roles would be taken over by higher teched super cruisers, which while they didn't match them in size and firepower, were more efficient in it's application.
Uparmouring and upgrading battlecruisers, especially at the expense of speed, tends to turn them into intermediate fast battleships, or if you try to reconcile Traveller, light high tech battleships. It should be noted, that means I see it viable to increasing the actual tonnage of the hull.
The Dunkerques and the Renowns were likely the only true battlecruisers at the start of the Great Patriotic War, and by the start of the Great War, the Splendid Cats had hit the sweet spot in nineteen ten, with eight thirteen and a half inchers, and weighing in at twenty seven kay tonnes.
While the Imperium might have commissioned battlecruiser classes prior to tech level fourteen, there are no canonical examples, and by the language used in books dealing with the Imperium Navy, they'd be reluctant to do so.
Likely, any remaining battlecruisers, outside of the Diasporas, would tend to be refurbished tech level twelves hundred kay and under, perhaps uparmoured to turn them into light battleships.