Core Question:
Is restricting a warlock's familiar to not be able to fly (or turn into any animal capable of flight) too much of a nerf to its functionality?
Background:
I'm about to DM a new homebrew campaign. I'm playing a lot with some of the more extreme functionality of D&D and warned my players that to fit in this campaign they might be required to make some early-game sacrifices in terms of character design, in trade for exciting payout later on. They were very on board at the time and quite excited about my proposed changes.
(To get in the right mindset, imagine something like a campaign where in session one, as early-level characters, everyone draws three cards from the Deck of Many Things. I'm planning something similarly crazy.)
One of my players has approached me and wants to play a warlock with Pact of the Chain. I've yet to DM for or play one, so I took a look through their class and got hung up on the familiar. Because of the story of my campaign it will be impossible for her to have a familiar that can either fly or that resembles a bird; it ruins half of my plot. I brought this up to her and we ended up in a rather heated discussion about whether I was nerfing her unduly.
I am okay with swapping the bird/flight form (most seem to have something like an owl, raven, bat, etc) for a different ground-based species.
She believes removing the option of flight nerfs what the familiar can do for her too much, and I don't understand the use of a familiar enough to feel comfortable laying down the law with certainty.
So back to my question; is removing a familiar's flight-based abilities and any stats one could use by assuming the shape of a bird a large nerf to a warlock's familiar, or is this no big deal?
Best Answer
Yes, it would be a noticable nerf.
Mechanically, having a familiar capable of flight - such as an owl - can indeed be rather useful, especially for scouting purposes - which is one of the main benefits of a familiar, barring RP purposes (RAW, familiars should be unable to carry anything noteworthy, so that won't be an issue).
Hence, disallowing flight for your warlocks familiar would indeed be a huge nerf, especially since familiars are generally not very useful in combat either, and you would be taking away their main out-of-combat non-roleplay purpose.
Therefore, if you can't convince your player to choose a different pact boon for her warlock - who, btw, won't have a familiar until level 3 anyways - you could allow her to choose a familiar that for some reason can't fly until the warlock reaches level XY, if that's an option you're ok with.
Of course, the in-game reason would not be tied to the level, which would be too "meta", but instead, a raven familiar could be too young or physically impaired to fly. Around the time when the warlock reaches the desired level, the raven has grown up enough or has healed, so the raven can now fly.
Also note that all creatures that are on a Pact of the Chain warlock's extended familiar list are either naturally capable of flight, or have means of shapechanging into a creature that does.
Side note
I also recommend checking out the dark wanderer's answer, which lists other reasons why you shouldn't disallow flight for your player's familiar, other than just targeting the mechanical implications of flight for a familiar, as my answer does.