Find Familiar doesn't take up a spell slot, does it? Pretty sure they just do out of combat casting for rituals then.
This is kind of an odd case as in P&P casting the spell with a slot takes an hour so in practice everyone takes 70 minutes to ritual cast it instead. Giving Pact of Chain Warlocks the spell at will is an odd hack to get around lack of ritual casting and how badly it would nerf PoC if it cost a spell slot (also familiars should persist after rests really).
