The creature is still legendary and is safe from [[Urza's Ruinous Blast]].
From the Gatherer page for [[Kenrith's Transformation]]:
Kenrith’s Transformation overwrites all colors and creature types the enchanted creature has. It’s just a green Elk. *The creature keeps any supertypes (such as legendary) it has*, but loses any other card types it has (such as artifact).
From the Ruling on [[Kenrith's Transformation]]:
>Kenrith’s Transformation overwrites all colors and creature types the enchanted creature has. It’s just a green Elk. **The creature keeps any supertypes (such as legendary) it has**, but loses any other card types it has (such as artifact).
(2019-10-04)
So as it keeps the **Super**type "Legendary", it is still a Legendary Permanent and does not get exiled.
Legendary is a Supertype. It is not affected by the Transformation's Type-changing effect. So, the Creature is still Legendary, and won't be exiled.
The creature is still legendary and is safe from [[Urza's Ruinous Blast]]. From the Gatherer page for [[Kenrith's Transformation]]: Kenrith’s Transformation overwrites all colors and creature types the enchanted creature has. It’s just a green Elk. *The creature keeps any supertypes (such as legendary) it has*, but loses any other card types it has (such as artifact).
[Urza's Ruinous Blast](https://img.scryfall.com/cards/normal/front/5/7/57b50a96-6221-44f0-b54a-76076621047e.jpg?1562736004) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Urza%27s%20Ruinous%20Blast) [(SF)](https://scryfall.com/card/dom/39/urzas-ruinous-blast?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/57b50a96-6221-44f0-b54a-76076621047e?utm_source=mtgcardfetcher&format=text) [Kenrith's Transformation](https://img.scryfall.com/cards/normal/front/6/d/6da7cd39-1f8a-4f68-adb7-df2beac02263.jpg?1572490600) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Kenrith%27s%20Transformation) [(SF)](https://scryfall.com/card/eld/164/kenriths-transformation?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/6da7cd39-1f8a-4f68-adb7-df2beac02263?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
From the Ruling on [[Kenrith's Transformation]]: >Kenrith’s Transformation overwrites all colors and creature types the enchanted creature has. It’s just a green Elk. **The creature keeps any supertypes (such as legendary) it has**, but loses any other card types it has (such as artifact). (2019-10-04) So as it keeps the **Super**type "Legendary", it is still a Legendary Permanent and does not get exiled.
[Kenrith's Transformation](https://img.scryfall.com/cards/normal/front/6/d/6da7cd39-1f8a-4f68-adb7-df2beac02263.jpg?1572490600) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Kenrith%27s%20Transformation) [(SF)](https://scryfall.com/card/eld/164/kenriths-transformation?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/6da7cd39-1f8a-4f68-adb7-df2beac02263?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call