Once you hit level 40 in Assassin's Creed: Origins (or 45 or 55 with the DLCs), a new item called the Tablet of Knowledge will appear for sale in all shops in the game. This item costs 1,000 Drachmas, and by buying it, a total of 1,000 EXP will be applied to your character.
This means that you can buy additional EXP at a 1:1 rate with Drachmas after you have hit the max level. If you want to gain additional Ability Points, then this is a great way to do it. If you got a lot of in-game money, you can pretty much buy a level or two every once in a while.
Some users have reported that there is a bug with buying these, and that the EXP does not get applied. You can see if this affects you by looking at your EXP bar while buying and, and see if it moves or not. If you are affected by the bug, then you can also confirm this by going to your inventory and see if the Tablets of Knowledge are found there, since they will disappear once the EXP has been applied to your character.