I suspect you misheard the announcement.
They didn't! It's terribly worded.
Many of our services only accept contactless or Oyster cards for part of their route, so please ensure you have a valid ticket for the entire journey you are making.
I first heard this back at the end of November, after which I contacted GTR to mention how this could be interpreted very differently to intended, to which they replied:
I hope my email finds you well, and thanks for getting in touch regarding the wording of the announcement about Contactless and Oyster payments. I can imagine the confusion this can cause, especially for those who are not regular travellers.
I have now passed the details over to the PIS team to see if this can be amended to save any confusion. However, this is not something that can happen overnight, so your patience is appreciated.
The first time I heard this, I also interpreted it as saying that
only contactless/Oyster were valid (and other tickets were not) on some parts of the network rather than contactless/Oyster only being valid on some parts of the network.
I believe I saw Matt, who records this and various other GTR announcements and manages the TL/GN PIS systems, state that they plan to re-record the announcements "in a few months" at the end of last month due to feedback from passengers (presumably not just me!).