Last year, he was only in recharge events only and after that he stopped showing up at all for a few months. Then sometime when great tree became to show up a lot, Hokage Minato reappeared again as a redeemable and started to appear in recharge events and lucky board/missions then became to show less frequently then he started to show up in RNG events where you could have gotten 80 frags. If you start to see this trend and any other ninjas. they will disappear from events so they can give room for newer ninjas. This doesn't mean they won't be gone forever but you can't expect it to show up quite often as you expected.

This is why the next time you see him as a redeemable, you better have enough to finish or get the full 80 frags since he won't be appearing as often as someone a little new like kakuzu earth grudge

Well the 1st half of 2017, I saw him only appeared in recharge, nothing else besides the one time in treasure of the sages. The point it was very hard to get hokage during those times. It became a lot easier when he appeared in redeemable events and more so RNG events like lucky star.