Yes, it is better to use a liaison here.

• non à faire, non attribué, non apparent, …
• non établi,

There is no modification of the nasal vowel : nɔ̃ nɑ fε:ʀ

The liaison is optional after non and rarely done.

Here is then its mainstream pronunciation :

La philosophie consiste à dire et non à faire. /nɔ̃.a.fɛʁ/

In careful formal speech, the liaison might be done and the rule is then to drop the nasalisation (but usage vary and the nasalisation might stay):

La philosophie consiste à dire et non à faire. /nɔ.na.fɛʁ/

There are however a few cases of more or less fixed expressions, or at least expression where non is strongly linked to the word that follows, where the liaison is always done¹ and the nasalisation sometimes preserved, the idioms nul et non avenu and non-événement (Thanks to @Personne for the example).

Here is an example with non essentiel where both the nasalization and the liaison are present:

https://twitter.com/ParisPremiere/status/1339307812367691776?s=20

and another one where non essentiel lose the nasalization and keep the liaison (1’00):

https://www.franceculture.fr/emissions/carnet-de-philo/carnet-de-philo-du-mardi-15-decembre-2020

As a standalone phrase, non à faire is slightly odd. We’d rather say Pas à faire where the lack liaison is also becoming the standard.

Finally, liaisons is a hot topic. Usage evolves with time and might also vary depending on the region or the social category of speakers so liaisons might move from mandatory to optional, from optional to forbidden and reciprocally, and among the optional liaisons, opinions might differ.

¹_{Surprisingly, the LBU states the liaison shouldn’t be done in non avenu, but I have never heard it pronounced that way.}

From my personal opinion, I would say that everything is possible. I’m not speaking here about how it should officially be, but about what I say and hear around me as a native speaker.

First of all, the liaison is here totally optional, and is generally omitted. For example in the slogan "Non à la réforme" that you could find in demonstrations or so, I do not know a lot of people saying the liaison here. It would be the same in your example "non à faire".

But the liaison is necessary if "non" is linked to the next word by a "-". For example, in this pandemia time, we hear very often the expression "non-essentiel", when speaking about theaters, cinemas or restaurants. In this case, I have always heard the liaison.

For the denazalisation, on my own experience once again, it is really 50/50. There is people doing it for "non-essentiel", and others no. I do not think that there is a version more common than the other one. Even personally, it depends. I pronounce it sometimes with the denazalisation, and sometimes without.

I don’t believe I’ve seen anyone answer with this so I’ll provide my own input as a native French speaker : it depends on your context (boring, I know)!

1. If you’re using « non à faire » as an adjective (« this thing is not to-do »), then it would have a liaison.
2. If you’re using it as a negation to a previous statement (« this thing is optional, not to-do! »), it would not have a liaison.

This is because you might as well add quotation marks to the words « à faire » in the second case: it’s somewhat unlinked from the « non », making a liaison less natural and the sentence less understandable.

My question came from this quote: La philosophie doit apprendre à vivre et non à faire des discours.

From all the answers and comments, it seems that liaison should generally not be made in this context.

Thanks for the input.