Again, this is only with champ shards. I don't think they are changing orange essence and skin shards.
That seems to be incorrect.
As far as they have stated orange essence will also be gained less if you don’t disenchant skin-shards now.
https://support.riotgames.com/hc/en-us/articles/115012640908-Preseason-Update-FAQ
IP & Blue Essence
Are you making any changes to the way disenchanting shards works?
Since you'll be getting more shards thanks to the new leveling system (and with other recent changes to honor and events), we'll tune down the relative value of the essence you get from disenchanting shards of all types—champ shards, skin shards, and ward skin shards.
In the current system, for example, disenchanting a champ shard will get you about a third of the BE you need to buy a champ at that shard's tier. In the new system it'll get you 20% of the way there. Also, with so many more champ shards in the system, we need to remove the ability to reroll them into permanent champs—but we’ll occasionally sell mystery champions for blue essence. For these reasons, you should go ahead and reroll or disenchant any shards you don't want to upgrade before preseason hits so you can get the most essence possible out of them.