Diablo 4 Mythic Runes are hiding behind an unexpected Runeword trick
Mythic Runes are a thing, but they have no additional effect or bonus.
Diablo 4 Season 15 has a strange hidden interaction that can turn ordinary Runes into their Mythic versions, and we are only now piecing together how it works. This does not involve every rune, only the new ones used for Runewords that were added in Season 15.
Runes such as 
Amn, 
Eth, 
Mal, 
Ort, 
Tal, 
Sol, 
Ber, 
Ist, 
Ith, can appear with the Mythic quality normally associated with Mythic items. However, these Runes do not seem to have their own drop source, and their effects are not really stronger than the regular versions.
So how do you even get these Mythic Runes, and what benefit do they have?
How to Get Mythic Runes
First you need a few of Season 15's Runewords. After crafting a Runeword such as 
Enigma or 
Infinity, you can use the Horadric Cube's "Upgrade Direct Item to Mythic" recipe to turn it into a Mythic version.
That Mythic Runeword can then be salvaged.
When you salvage those, the Runes returned from the Runeword can come back as Mythic-quality Runes instead of their regular versions. Keep in mind that you do not necessarily get every Rune back, and they might come back as normal Runes.
There is one weird detail to know as well. The Mythic quality seems to also be linked to the Rune stack. Players testing the interaction found that placing regular Runes onto an existing Mythic Rune stack simply turns the entire stack into Mythic as well.
Mythic Runes Are Not Stronger
There are multiple problems with this. First, you need to craft Runewords first, and then you need to spend your valuable 
Resplendent Spark to turn them into Mythic items before salvaging them for a chance to get Mythic versions of the Runes. And second, despite their purple quality, Mythic Runes do not currently appear to provide any additional bonus.
The Mythic version still has the same Offering cost and effect as the regular version.
So Is It Worth It?
Let's be honest, not really. The current crafting loop for this would be:
Use normal RUnes to craft a Runeword → upgrade it to Mythic → salvage it → receive some of its Runes back as Mythic Runes → use those to craft new Runewords in hopes it directly drops as a Mythic Runeword.
It is really not an efficient way to obtain Runes or Runewords. Upgrading a Runeword to Mythic requires spending valuable resources, and you should not burn through Sparks just to collect purple Runes that, in the end, do not provide anything else.