Wordpress.com offers 100-year plans. Hosting + domain is $38,000 or domain only is $2,000.

https://wordpress.com/100-year/

Simpler is better. Ideally, it should be a static site, and hostable on any domain.

Failing that, choose technologies that have been around for a while. PHP, Ruby, and Java have been around for 20+ years, and are still going strong. There is no hope that anything touching Node or npm will run in a year.

1. Defend against format obsolescence. Prefer plain-text formats, or at least ones that can be mostly-understood by humans, like markdown or semantic XML. (And not, say, PDFs.) For audio-visual stuff, prefer the simplest kinds of highly standardized and common formats.

2. If you need a website, prefer a static site generator. If you need a dynamic site, periodically export a static version.

3. Don't count entirely on the hosting service, store offline copies (as a standard zip file) alongside other content of interest to heirs, such as a will. Distribute redundant copies to relatives.

20 years - google doc with backups in your email and wherever your taxes and medical stuff is, and printed copy with your home records

40 years - print and bind the google doc in 20 years, store it with their stuff when they leave the house.

60 years - publish the book buy a bunch of copies and distribute

100 years - it needs to be a very good book

You might be interested in Arweave or IPFS:

Arweave network is like Bitcoin, but for data: A permanent and decentralized web inside an open ledger. [0]

The InterPlanetary File System (IPFS) is a decentralized protocol, hypermedia, and peer-to-peer (P2P) network for distributed file storage and sharing. The shadow libraries Anna's Archive and Library Genesis host books via IPFS. [1]

[0] https://www.arweave.org/

[1] https://en.wikipedia.org/wiki/InterPlanetary_File_System