Is there any prerequisite for meeting Thanatos?

You've been very very unlucky. According to the wiki page:

Thanatos occasionally appears at the beginning of regular encounters and challenges Zagreus to see which of them can slay more shades.

There's no pre-requisites for him to spawn, but once you've encountered him in one of these runs you can find him in the House of Hades:

Thanatos can be found in the same hallway where Achilles keeps guard.

The wiki page doesn't cite any chances, but I suspect you've just been one of the super unlucky players that has never found him.

Source

We now have an official answer from the developer to this question, so I'm putting it here:

It should not be necessary to successfully clear the game to get Thanatos to appear. You just have to run into him in Elysium.The requirements are:

• Have met Alecto or Tisiphone (who can appear after defeating Meg several times)
• Encounter Thanatos in Elysium (about a 1 in 10 chance per chamber)

Subsequently he will able to appear in Asphodel or Tartarus as well. (Note I'm spelling out the requirements this directly only because of the problems being reported here.) The 1 in 10 chance is purely random so it's possible to get unlucky over long periods of time with regard to his initial appearance.