Participle agreement

If the English text had been "that she had never felt before" rather than "such as she had never felt before", then the conjunction would have been "que" instead of "telle que", and I think there would be no need for "en". In this case the COD would be "sensation" and the agreement would be required. But in this case the COD is "en".

The agreement (or not) when en is the COD seems to be very complex, even for native speakers. You can read the following threads if you're interested, but the best advice seems to be:

Quant à l'accord avec en, je l'ai déjà dit et le répète: le plus simple est de toujours laisser le participe passé invariable. (L'invariabilité est en effet toujours correcte, contrairement à l'accord qui n'est pas toujours possible.)

https://forum.wordreference.com/threads/plus-despoir-dexcitation-et-de-peur-quil-nen-avait-jamais-ressenti-s-accord.2288492/

https://forum.wordreference.com/threads/en-accord-du-participe-pass%C3%A9.537058/

https://www.etudes-litteraires.com/grammaire/accord-participe-passe-en.php

Une sensation de liberté telle qu'elle n'en avait jamais ressenti. -- A feeling of freedom such as she had never felt ("of them") before.

The en refers to all the feelings of freedom, of which (=en) she had never felt any before. It is superfluous in English but often used in French. It occurs in sentences like:

Je n'en ai jamais vu aucun. -- I have never seen any (of them).
Oui, j'en ai ajeté trois. -- Yes, I bought three (of them).

Se ressentir is an intransitive verb -  se ressentir de + qch/inf  being the full verb

telle que  is really a conjunction and que is not a relative pronoun 

hence elle n'en avait jamais ressenti'  means "she never felt it"  "en" meaning  "of it" and does not make the part particple agree with it.

Thank you, R, that is very helpful