It seems to have disappeared indeed, though other FIP stations (FIP Jazz and the like) are still there.  To add it manually:

On desktop app, Manage → Add Radio Station...

Streaming URL:  http://icecast.radiofrance.fr/fip-midfi.mp3

Station name: anything you like, but might as well call it FIP.

This will make it appear in Radio by TuneIn → My Radio Stations.  From there, you can add to your favourites of you wish.

In addition, I have just been told by a friendly Sonos rep that due to some licensing issues some stations have disappeared from TuneIn but are available on the MyTuner service.  Add this service to your account (Manage → Service Settings) and then search for FIP and add it to your favourites.

Thank you Jem - that has worked brilliantly. FIP is one of my favourite stations and I would hate not having it available. Once again many thanks. Ian.

Thanks Jem - works now for me too

Jem, you absolute STAR! FIP is one of the go-to stations in our house and we've been bereft since it stopped being a service on Sonos no thanks to Tune In. What pisses me off is that there is no announcement, from either Sonos or Tune In, it just stops! And then you have to go and do your own detective work, I couldn't even find any info via an online search. Thanks again. 

This has been driving me nuts too!! Thanks Jem

your instructions didn’t quite work for me on my iPhone 

I went to browse in Sonos app, then clicked on the tunein icon

Then radio stations, then the 3 dots in top right hand corner

in my radio stations> add new station & from there pasted in your link.

for some reason couldn’t find anything in my app saying manage>add radio station
 

But thank you- the other fip sub-genres can’t compete with the eclecticism of the main station. Hands down the best radio station in the world.

Thanks SO much Jem and ianbudgie.  I was, like many posts I’ve read, feeling bereft at loosing FIP, like many others I just couldn’t understand what had happened to it. So Jem thanks SO much, but then as I control my Sonos app through my phone I then thought Ok so how do I do this on my iPhone and then saw Ian’s post… as I type this Fip is playing and I’m smiling again… SO HUGE THANKS.

Many thanks for the helpful posts - like others our FIP just left the building about 2 months ago! We’ve managed to get it back thanks to ianbudgie’s tip but it’s not showing on the radio statio list (and so disappears again when we switch stations). If anybody can help with something we may be missing we’d be very grateful.

This thread may help too. 
https://en.community.sonos.com/music-services-and-sources-228994/tunein-blocking-foreign-radio-stations-in-the-uk-master-thread-6848128