1. C language.
2. *nix tools (shell and friends).
3. SQL.
4. Basic IPv4 networking.
These things I learned around 20 years ago, they didn't change much and they are useful for me to this day.
The point being that sometimes the tools themselves don't need to survive because you take the lessons from one thing to another (e.g. move semantics and rust/modern c++)
[1] - https://pragprog.com/titles/btlang/seven-languages-in-seven-...
Assuming you mean for a VPS, the majority of providers provide IPv6, and a good many would advertise your IPv6 prefix and route it to your box for a nominal fee. It's far cheaper to do anycast IPv6 than IPv4 because of the cost of IPv4 address space.
If you actually meant on-site for a business, that again depends on your provider, but again most providers should be able to give you a static IPv6 prefix from their own range if they're able to provide you a static IPv4 address. If not, you can always tunnel IPv6 to your site.
Doesn't mean it stops working or being useful, and in arguably IPv4 case it is still of far more importance than v6.
Probably depends a lot on a particular location.