Some thoughts:
1. I have no idea (and am curious) how frequently ToS are updated and how often the updates are meaningful to me as an end-user as is the case with Zoom and NightOwl. It would be really interesting if you post some stats after running this service.
2a) Based on the Reddit example: 9 blocks were changed and 1 block was deleted and in a hypothetical use case where I get an e-mail notification about this I would have to read them all.
After reading them none appears substantially different as far as I am concerned as a Reddit user which makes me wonder what the SNR of this service would be as compared to deferring to the HN front page to notify me of major changes that would potentially change my usage of a product.
If I'm missing something significant in this example it would raise the issue of my capability to accurately interpret these changes and therefore whether such notifications are relevant.
2b) Perhaps the "Guidelines for Healthy Communities" changing to "Moderator Code of Conduct" could be significant to a moderator but the details aren't included in the ToS and on searching are listed in a separate document. I wonder how often a ToS reference terms in or includes agreements to other documents, presumably with this service I would have to add and read each one separately?
Overall, it's a great idea but I'm very curious how useful this will end up being in practice, if the SNR is low I'm unlikely to read all the diffs.
For what it's worth I think the price is very fair and I chipped in to support the initiative using my "spam" email. It would be really great if you could do a write-up after a while with some numbers regarding the comments above.
I assume (but IANAL) that there may be some legal liability or at least ethical risks to account for if you were doing this but I would be willing to pay more for a reliable summarized service (i.e. not an off the shelf LLM interpretation) that I can trust to notify me of potentially relevant changes of comparable quality to the HN hive-mind with the advantage of being able to add the services I personally use that the collective here may not.