Alexios was never meant to be playable.
www.pcgamer.com
Ignoring the PCgamer spin here are the facts:
Assassin's Creed Syndicate was originally going to feature a more even split between its two playable leads, twins Jacob and Evie Frye. After meddling from executives, including
recently-resigned Serge Hascoët, the balance became much more skewed in Jacob's favour. In the final game, it is a rough 40/60 split between Evie and Jacob
Assassin's Creed Origins. The original story saw protagonist Bayek killed off early in the game to be replaced by his wife Aya. In the final game, Aya has a massively reduced role.
Assassin's Creed Odyssey was going to originally only feature Kassandra as its player character. Hascoët, and unnamed people within Ubisoft's marketing department, claimed that games with women leads wouldn't sell, and so Kassandra's brother Alexios was included as an alternative lead .
Aforementioned executives have now all been canceled due to harassment, so expect Ubisoft to have full creative freedom from now on.