Just saw that Tye beat me to it but here's my two-cents as well! Very much in line with what Tye provided. ^^
There's been some back-and-forth about this over the years, but here's what I feel the general consensus has come to:
In the briefest of summaries, if a model is not claimed in the Official Model List, then they're free for anyone to use. So a model suggestion for an adoptable (especially an unclaimed one) is fair game for anyone to use. Generally, I would say this applies to archived characters' models because they are not actively played at the moment and thus cannot be officially claimed.
However! There's kind of an unspoken rule of courtesy that many of us follow in the event of a model chosen for a WIP (especially one with a good bit of sheet work done) and for certain cases of archived characters.
For the former, if I see that someone has a good amount of sheet work done and has chosen a model, I personally mark it "off-limits" in my mind. There's quite a bit of gray area in this regard but personally I've found that this way works pretty well and minimizes harsh feelings.
For the latter, if the driver has other active characters, I think it's a good idea to shoot the driver a PM and touch base with them as a courtesy - good example of this is when I talked to Fee about using Nina Dobrev for Astoria after she had archived Vassilisa. But if the driver is long-since inactive, I wouldn't worry about it.