Where is Hextech Mystery Champion Shards?

Yes.

The original one existed as a means for people to use IP to get shards they could disenchant for BE in order to level up mastery (or occasionally get lucky and get a shard to directly level their mastery).

Now people get plenty of shards and direct BE, so they weren't necessary. (Champion rerolling was also removed since it wouldn't be balanced in an environment where some people might get literally 5 shards from a single level up if they were cheap shards.)

The more recent one gave champion permanents rather than shards, and only existed as part of the BE emporium which has since expired. (It'll likely be back a couple times a year.)

Riot seemingly needs to make even more money so they're removing or worsen everything that made it somehow easier or cheaper to get anything (Champion Rerolling, less "IP" aka Blue Essence but a lot of stuff got much more expensive i.e. Champion Masteries, nearly 1/3 of the Orange Essence for disenchanting even tho you have no other way to get it besides chests so far, those emotes that are now also part in the chests so you're chance to obtain good skins chards shrunk even more and so much more)

[sg-lux-2]