I gave this card a light surface cleaning. It was almost twice as dark as it is now. Just a light brushing with a q-tip and water, doing a small area at a time then drying. I didn't go farther since the surface coating is a bit crazed (A network of tiny cracks from the coating shrinking) and the dirt is into the cracks in many places. It might have cleaned better by soaking, but I was chicken.
It's sc350-460 f25 by the way.
A couple notes that apply specifically to T206s and somewhat generally to many other cards.
The surface of the color side is coated stock and won't absorb water well enough to be damaged. The coating is also water resistant and holds to the paper and ink well. Other series may have different paper or a gloss coat that can be damaged by water.
The inks used in lithography are all oil based and will not be affected by water. The colorants might be, but it's nearly impossible to know what exact colorants were used. Assuming the typical ones for the era most of them are totally unaffected by water.
Inks used for other processes may not be oil based. That's part of why knowing the process matters. (Some old british stamps are printed with what are called fugitive inks that dissolve in water. One series is "doubly fugitive" meaning it dissolves in both water and solvents.)
A gloss coat may be oil based, or alcohol based like shellac. Most oil based glosses aren't bothered by water at all, Shellac is and will absorb water and get cloudy- Like rings left on old furniture, which is why we use coasters.
The papers all begin as a slurry of stuff in water. The machine then scoops it up on a screen, spreads it evenly and dries it. Some papers will absorb a lot of water and expand, like 70's Topps many of those have a high wood pulp content. The paper used for T206s probably has a high rag content, and is far less absorbent. (And may actually be nearly acid free. I just don't have a sample I'm willing to test since I'd have to destroy a portion of it.)
And that's an "amateur" take on the technical aspects of cleaning/soaking.
Steve B