I realize that a bug like this is very difficult to track down. Best of luck to anyone who wants to try to find it!

I found a mod conflict that causes the Pikeman unit to be removed from the Technology Tree, and cannot be trained after Military Tactics technology is completed.

Here are the (very limited) steps I took to track down the bug:

1) Before activating Crazy Barbarians, Pikeman are avaialble as normal. Many mods are active simulatenously, including nearly every mod by JFD and Sukritact, plus the "Moar Units" mods.

2) Enable Crazy Barbarians mod--> this shows the symptom, and pikeman become unavailable.

3) Disable all mods, and enable only Crazy Barbarians and first-party DLC. --> the symptom goes away, and Pikeman units can be trained normally.

If the issue could be fixed, that would be excellent, but I'm guessing it would be very challenging to track down the root cause.