HUMBLE: "Please save this email for whenever you’d like to access it again."

I'm sorry, I don't know, but have a bump.

I guess they're trying to improve security mesurments by going one step back?

I don't know about your question exactly (I know you can claim entire bundles when one is purchased as a gift) but try guessing or bruteforcing one of 48,453,916,488,902,607,769,120,106,730 possibilities.
That's more than fifteen thousand trillion centuries at 1000/second (this is an online attack so even that's unrealistically fast).

Yes, gift links from bundles are still accessible for anyone via URL, even if you claimed one.

Has anyone tried brute-forcing a specific Humble URL? I haven't but i'd bet a snickers, after ~ 20
attempts that lead nowhere the IP Adress gets a IP ban - cooldown of whatever time-length.

Judging the URL's though, 16 digits of (small, capital letters + numbers) "guessing" is nigh impossible combined
with an IP cooldown. https://howsecureismypassword.net/ < try entering here some hb-url-ending you've claimed safely.
That is why they probably have left it like this.

I just claimed all of my purchases to my humble account a couple days ago. I'd never bothered logging in when purchasing, as it didn't seem to matter since those emails claim you need them for access. Now I can get everything by logging into my account on humble and clicking Purchases. It's all there.

You use to be able to claim gifts to your account, but as you said it no longer seems to be the case.

who gives a shit, just redeem the steam key and you can access your game whenever you like

If someone generates a gift URL from a bundle they bought and sends it to you, it only sends you the Steam key and doesn't include the DRM free download, so there's no need to redeem it to your account.