For a Pact of the Blade Warlock that can "transform one magic weapon into your pact weapon" (PHB. 108), the weapon can be dismissed into an extradimensional space. If the weapon is magic and requires attunement and is dismissed for over 24 hours, will the attunement to the Warlock be broken with the rule "A creature's attunement to an item ends [...] if the item has been more than 100 feet away for at least 24 hours" (DMG. 138)?

Your DM will have to make a ruling on this since the rules don't say

There are no rules on how to measure the distance between one dimension and another which makes sense because even the concept of trying to do so is pretty much nonsense. There simply is no defined distance between the two and maybe not even a way to measure. Certainly there is nothing about it specified in the rules though.

Thus, as with all rules ambiguities, your DM will have to make a ruling on whether they will count extradimensional places as being further than 100 feet away or if they consider that is does not count. It is also worth noting that this ruling would also apply to any other extradimensional storage such as a portable hole or handy haversack.

However, it really shouldn't matter which one your DM chooses in the vast majority of cases. It is trivial for a warlock to summon their weapon and as they do it once a day the issue won't ever come up. As an adventurer, it seems unlikely that they would not already be meeting or exceeding this.

The DM could go one of two ways. He could say the Warlock is the dimensional anchor for the space, and thus the sword is 0 feet away when it's in its extradimensional storage. Or that the dimensional anchor is the patron and thus the blade is sent across the planes.

That being said, there are so many interesting ways to work daily summonings into the RP. A daily ritual to the patron using the blade. The warlock could have a delusion that the sword is sentient or possessed by a loved one, or needs to be summoned as much as possible, giving a character arc as the warlock is forced to confront their obsession or delusion. It could even be that the power of the attuned weapon acts to preserve the attunement and will self-summon, and can lead to surprise encounters. All these can be used to develop the warlock's relation with his/her patron.

Yes, it seems implausible to argue that “on another plane of existence entirely” isn’t more than 100 feet away (though, technically, the “distance” between them is undefined). So it behooves a Pact of the Blade warlock to summon their Pact Weapon at least once a day to maintain that attunement.

If this text is accurate:

You can transform one magic weapon into your pact weapon by performing a special ritual while you hold the weapon. You perform the ritual over the course of 1 hour, which can be done during a short rest. You can then dismiss the weapon, shunting it into an extradimensional space, and it appears whenever you create your pact weapon thereafter.

"Whenever" doesn't say anything about a 24-hour fizzle. It says plain that when you transform a magic weapon, that means whenever you "create" your pact weapon, you get that.

You can’t affect an artifact or a sentient weapon in this way. The weapon ceases being your pact weapon if you die, if you perform the 1-hour ritual on a different weapon, or if you use a 1-hour ritual to break your bond to it.

It specifically enumerates the list of ways you abandon that pact weapon. 24 hours of inattention is not on the list.

The weapon appears at your feet if it is in the extradimensional space when the bond breaks.

You get the underlying magic weapon refunded to you when it ceases being your pact weapon. So if this 24 hour rule was applicable, you're riding a horse when the 24 hour window expires and unbeknownst to you it appears on the ground under your horse, and you ride away from it. The next time you summon your pact weapon, you get a generic one. ... This doesn't feel like it corresponds to the effort to create one, or the enunerated ways you abandon the pact weapon.