Most of the time, communication is good, but there are some occasions where it isn't needed and can cause problems.
I think this is one of the latter situations.
Neighbour could have told you they planned to put spikes on their own fence and why. It would have involved mentioning your cat. They probably chose not to because they felt it would infer some criticism of you - in terms of it being your cat that is messing in their garden. They probably recognise you have no control over the cat, so don't want to imply criticism of you - so thought it best to just do it and say nothing.
Given it is their fence, it is perfectly reasonable.
Should you now bring it up? Probably not, because to do so implies criticism of them for not mentioning it before. Are they entitled to try to prevent your cat going in their garden - well yes. Are you entitled to have a cat - well yes.
Your cat isn't loved by them and their spikes probably aren't loved by you. Do you both accept the right of the neighbour to have them - probably yes, so in this case it's best to just carry on as if nothing had happened.
It's not the same as for example, neighbour deciding to put up an extension, or have a loud party or have a big bonfire on a nice day - these are all things which common courtesy would suggest neighbours speak to each other about.
This is a bit different, and I think best just ignored. You might find neighbour later mentions the spikes (probably rather awkwardly) and it would be good at that point to be gracious and just say you understand.
They haven't done something wrong and neither have you. Sometimes we need to do little things to make living with neighbours nearby more bearable and accept others might want to do those things. They aren't telling you to get rid of your cat or to control it in a way you can't or trying to kill your cat, but to deter it. Hopefully you can understand why, but also that it might be a bit awkward to talk about.
Move on.