From what one of the devs stated yesterday it sounded like they experimented with this early on, but due to technical issues, they've had to shelve it for now, although they're interested in circling back to it later.

This was me, and that is correct. I am personally still hopeful we'll get to it at some point, but it was technically unfeasible, a UX nightmare, a minor stability concern, and did deeply unpleasant things to the AI.
In the end, we had to choose between "do we want to have travel matter most of the time?" and "do we want to have travel matter absolutely all of the time but risk delaying 6 to 10 months (if we could even get permission for that)?". Unfortunately, a good implementation in most places generally beats a perfect implementation in every place, so that's what we went with.
It's a colossal shame because it was really rad watching commanders have to trek back after crusades or long foreign wars, or just when dismissed from the front, but the knock-on problems just weren't worth the gain.
I think this perfectly fit to this. I understand that for comon war this would be a mess. But for crusade and co, the journey itself was something more important than the battles.
And we could during the traval gather more levy, convice lords to join the crusade or plan for personnal plan (like the sack of constantinople in 1024).
I 100% agree with the spirit of what you're saying, but per my above post, it was not technically feasible.
I would like to see long-range wars being more of an expedition at some point, but that'd likely have to wait for either some kind of warfare-themed flavour pack or a religious expansion (which would presumably cover the Crusades).
I think you're confusing the verb "travel" and the "travel system" being added to the game. The travel system is much more than just leaving court, it is also things like choosing your route, your travel party and drawing events/making decisions along the way plus maybe more we won't quite grasp until we have it. Regents during war is probably a good thing, but I don't think that requires the whole travel system.
For general war I don't think the travel system as we know it makes sense. You don't know your route a priori, you're taking much more than a small traveling party with you and too many events may bog you down further than the comparitively large amount of micro required by a war alone.
For a great holy wars it might be different. Since regular wars don't work well for them currently and there's a lot of interesting things that happened with IRL great holy wars that can't be represented right now (and more importantly maybe could have fun gameplay). My concerns are do you still have the regular war after "attending the Pope's army gathering event" and how does that war work (Does the pope control all troops, are only a fraction of your troops committed to the fight? Idk, but maybe at least they could start in the same location for the war I guess) or does the whole "war" just play out in events? Also what is the counter play for the defender here? Can they manipulate the gathering to try to redirect the war? How do they defend? These ideas might be impossible in the current situation.
I would strongly echo this.
tl;dr: it isn't clear how wars fit into travel. Great holy wars might but there's still a lot of questions. We have to wait and see the nitty gritty details of the system and maybe try to mock up a mod.
Wars do not currently affect travel, I'm afraid. Or, well, they don't affect the travel of people
doing the warring - IIRC I think they can/do cause problems for people travelling
through war zones. Not constant but I believe there's some event content there, though please don't quote me on that. ^^' Big expansion, hard to keep track of everything.
probably some genius modder will add it back.
Sadly basically impossible, I'm afraid. Warfare is the least moddable thing in the title, outside of CBs - they actually have a macro over on the CK3 Modding Coop Discord for listing all the things you can't do with it. It's one of the title's older systems and it doesn't have quite the same amount of consideration given to moddability as many newer things (which, for warfare, is most everything else).
I think a travel system for the Crusades makes a whole lot of sense - think of all the adventures (and misadventures) Richard I of England had on the way to the Holy Land and back.
On the way out, you have the capture of Messina in Sicily and the conquest of Cyprus after a storm wrecked Richard’s fleet and his fiancée was taken prisoner by the island’s ruler, followed by his marriage there (note he subsequently took his wife on crusade with him as part of his entourage, though they returned to England separately). On the way back, you have Richard’s departure from a Corfu ruled by the hostile Byzantine emperor while disguised as a Knight Templar, another shipwreck leading Richard’s entourage to take a dangerous land route through Central Europe, Richard’s capture and imprisonment near Vienna by Leopold of Austria (who accused Richard of scheming to murder his cousin, as well as being personally offended that Richard had cast down his standard from the walls of Acre), leading to Leopold’s excommunication for detention of a crusader, and Richard’s subsequent ransom from the Holy Roman Emperor’s prisons (while his brother and regent John, together with the King of France, were trying to pay the Emperor money to keep Richard a captive!).
This seems to me to be a series of stories which are very well suited to CK3, and in particular the new travel system - without it, we’d simply move a stack of troops from England to the Holy Land and back again, and that would be that.
Agggh, this pains me. You're entirely correct, of course, but like I say, not something we have. Something I
hope we'll have some day, but adding travel to war in a systemic fashion
and providing content for it would likely've cataclysmically lengthened the DLC's production time, and I don't think anyone (from Paradox Leadership to the fans to the team themselves) would've wanted
that.

Another time, hopefully.
One good thing I
will say about not doing it at the same time as other travel stuff is that it's much easier to work on a finished system than an in-progress one, so whatever we use travel for in the future, we'll likely have an easier time of it.