Units do not disappear due to the onset of a period in which they become unavailable for training. But if you select a unit for training and the training is not completed when the unavailable period comes, then this unit will be removed from the training slot. Of course, this is only possible with units that require more than 1 turn to train - for example, the Varangian Guard. This is easy to see if you put this unit on training in 1320.
To make the Varangian Guard available at a later period in the vanilla campaign, you need to edit the file CRUSADERS_UNIT_PROD11.txt . I recommend keeping a copy of the original. Open the file, find the entry "Varangian Guard". Now find the entry "EARLY, HIGH" in the text. After "HIGH" enter ",LATE". Save the changes. There is another similar entry below in the text, but this is not for campaigns, but for individual battles.
By the way, there is also a unit "Varangian Guards" (in the file it is "EarlyVarangianGuard"). For some reason, the developers left him inactive - he is not available in the campaign for training and even as a mercenar(this, of course, is corrected by editing).