I thought about AWS MQTT "IoT" as the pricing contains gems like a million MQTT packets for a dollar. You'd use a MQTT "retained message", push the current SHA1 hash of the page or whatever will fit (maybe only a CRC32 LOL), then connect an hour later to receive that message containing the hash. The cost would seem to be $1 per 5 years because AFAIK AWS meters on input and output... I'm not bored enough to audit the AWS pricing structure to see if this will work. I think $2 per decade would be hard to beat for a billed service.

I think the cheapest "cheaty" way to store data for free would be to pollute tracking sites. Open an account at some company, change your mailing address every hour, etc.