Ah, you're talking about leaks and "shelved" content.
No region gets stuff "early" (NA and EU maybe first to get download the content, but all regions essentially get the same content on the same day), its just that someone at Riot goofs up, and some info on current projects gets out to the community. For example, the leak that contained Elementalist Lux, and the first batch of Star Guardian skins. It also had insights on to what Urgot was shaping up to be.
As for shelved content, Riot shelving skins when they hit the PBE is very rare. The last time this happened were the "Sewn Chaos" skins, and before that was the previous batch of SKT skins (they came back, but SKT Sivir was trashed completely, and replaced with SKT Kalista). Before that, I can't really think of any examples that large except for Dragonmaster Swain (which still could be a possibility), and Ao Shin (which eventually became Aurelion Sol).
Generally, shelved content pertains to gameplay. Riot can't test everything to the same degree the community can in the same amount of time. In fact, Meddler specifically said quite awhile back that they get more data on game changes within the first 24 hours of a patch going live then they do in all of their in house testing and PBE testing combined. The put "experimental" changes on the PBE to see how certain changes will act on a larger scale. For example: the Lissandra passive change, the Udyr mini rework, and a new type of ward. If they don't work, they pull them from the PBE, and work on them some more in house. If it turns out that they can't find something that works in house, they "shelve" it for a later date, and go on to work on more pressing projects (this is what happened to Lissandra's passive, and that unique ward).
The only way these upcoming skins will get removed from the PBE is if there is a large enough community outcry. "Sewn Chaos" didn't have that large of an outcry, but Riot felt the skins didn't come up to their standard of quality, so, they pulled them. The outcry over the last batch of SKT skins was MASSIVE. In fact, they talk about this in the "behind the scenes" video on the development on the most recent batch.
But, seeing as how the general consensus is over both arclight Yorick and Beekeeper Singed, I highly doubt any of these skins will be pulled (and, lets be honest, in the past 3 to 4 years, has there ever been an ultimate/legendary that has been underwhelming? I certainly don't think so).