To try to summarize a longer story, I had a fairly new (just turned 2 years old) smartphone break down last December. I had the Steam mobile app and Steam Guard on it, and I found it very convenient using that QR code reader to log onto devices like my Steam Deck.

Basically I changed phones twice recently, and I ran into a similar problem both times and had to go through customer service, both times.

I had data I wanted to recover from that broken phone, so I didn't want to trade it in and temporarily used an older phone for a couple of months. I had to ask Steam customer support to help me move the Steam Guard to the new phone once.

Then when I got my data recovered from the broken newer phone a couple of months later, sent it in and got it replaced under warranty, and I tried to activate Steam Guard on the new phone, I ran into another problem yet again. When I tried to add an authenticator for my phone number, a link was sent to my associated e-mail address. Then when I opened that e-mail and clicked that link on my (same) phone, IMMEDIATELY (less than one second later) I get an error message that just said this:

"The link you clicked has expired. Please try again with a new link from a new email."
With a button below that: "Try Again"

I repeated trying this five times. Then I gave up and contacted Steam customer support yet a second time in two months - to move Steam Guard again to the new device.
After a couple of exchanges back and forth, and me describing exactly what happened and submitting screenshots, a representative told me that my browser might be changing my link on me, but just to try again on different browsers and devices.
I was using Chrome and installed and switched to Firefox on the same device, which didn't work,
Then I just tried submitting the phone number on my phone app BUT opening the resulting e-mail and verification link on my DESKTOP PC instead (using Chrome), and THAT worked.

Now that I've given background for what happened, here is the problem I see may affect others in the community, and a trouble-saving suggestion.

It seems the immediate "fix" was simply to try a different device. But the error message that popped up was very unhelpful toward that, all it said was
"The link you clicked has expired. Please try again with a new link from a new email."

Which in my case, would be misleading - trying again didn't change anything. I could also have misread that as trying from a new email ADDRESS, or someone else very well could.
If a common solution is to simply try from a different browser (quite possible, I experienced that from a different website, resolved by simply switching browsers) OR in my case, try from a different device, the solution should at least suggest that. That would save both customer and support representative time.

"The link you clicked has expired. Please try again with a new link from a new email, OR try opening a NEW link from a NEW email, from a DIFFERENT web browser, or a DIFFERENT DEVICE."

Just a humble, easy-to-enact solution, while the software teams look into possible underlying issues and come up with solutions so that this error doesn't happen in the first place whenever customers change Steam Guard to new phones.