Can I date spoof to access special Wild Area raids after the event period?

I can confirm that the game keeps "memory" of at least the last "wild area news" you updated to.

Apparently, in the past that meant that if you played online and the game downloaded a specific event news data, you could keep that event active indefinitely by simply never letting the game connect to internet again. I can't personally confirm this info but posts like this one on reddit point in that direction. I don't think this is the case anymore - the game seems to check the event close date too.

Either way the real issue with your idea is that while currently the game does indeed seem to remember the last event you download (after all, it has too) it also seems to remember ONLY the last Wild Area News you downloaded. Take this as an example:

• October 1 to 3, 2021 - Alolan starter Pokémon
• September 17 to 19, 2021 - Lunatone/Solrock

If you downloaded the September 17-19 news, you can change the console date to that range and the event will be available.Yet, if you downloaded the October 1-3 event, you lost access to the previous one.

Also, please notice that I don't know what adverse effects time travel could have on your game. If you play on October and then change the date to some time before the last play date the game may detect that and/or not behave correctly. I wouldn't also exclude the possibility that simply accessing the game on a date past the currently stored event had the side effect to clear the event. So, the only safe use of this technique is for example to be able to play an event if you downloaded it and then couldn't play on Saturday/Sunday.