Random hardware suggestions, using mobile Linux support as a litmus test

• Pinephone (Pro): Main downside is that OG Pinephone has extremely anemic hardware, and the charging circuit is not controlled through hardware for some insane reason; hope the kernel devs of whatever OS you put on it knows how to not turn your phone into a bomb. Also Pine64 as a company has gotten flak for their support of Manjaro. Can't deny how good the price is though.
• Fairphone 4: Good hardware, but expensive. I don't own it, but it works good on postmarketOS according to the wiki.
• Librem 5: Overpriced compared to the earlier members on this list, but you can guarantee the phosh interface will work well considering it was developed by Purism as well.
• OnePlus 6 and 6T: I don't know much about these, but they're very popular with the mobile Linux crowd.

As for the pixel, there's work on it but it's still broken at the moment. As for the hardware being too old, I haven't used anything Android in a while, so I don't know how much performance degrades each release, but a mobile Linux distribution should run just as good today as it will 20 years from now, assuming you use the same interface.

Can someone explain to me under what circumstances would using an old phone be risky (under a common reasonable threat model)?

I think it's a bit too old, if you want to stay in the pixel ecosystem maybe try to grab a 6, 6a or 6 pro. They are around $250, and they are great!

4a is end of life already, so no firmware updates from Google. GrapheneOS has legacy builds available for it but doesn't recommend using them, and they might go away anytime soon

get a used device which is still properly supported, don't buy brand new e-waste

The one we can't talk about...

I don't get it ? Why can't we say it's name ?

Pixel 4a was one of the last in the Google lineup with a headphone jack (5a being last). The OEM lost its way after that. This enough to not recommend their devices as far as I am concerned.

Yes, that is too old for a new phone considering it's already past its end-of-life for both official support and your OS. I'm not sure why you'd recommend them to buy new either - a phone like that is only going to be good value if you pick up a used one for cheap. A new model will be massively overpriced for what it is (and may not even be new, just refurbished and repackaged).