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.

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.

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.