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Unknown capacitor(?) on IBM Thinkpad 770 DCDC board....

The palm rests only on the 38x systems have the rubber coating, but the cases don't. The coating on mine is VERY weak at this point, but not sticky. It will scratch with a fingernail, and an attempt to lightly use a magic eraser to clean up some damage caused by an old sticker caused it to start coming up and off.
You can see the damage in the lower left where the Windows sticker would have been. It was peeling on mine so I had to remove it.
The 600 series is the last that had bad coatings from what I recall from my hoard (mostly palm rests).
I think the X30s rot as well which are later than the 600s. I've had to seperate them.

When I've stored the ones that rot, I've placed them in a tall cardboard box and built wooden shelves inside so they sit with an air gap around them, with packs of that silica gel to dry out the inside of the box. If you store them touching anything, they rot, just like under the sticker on the palmrest.
Any rubber is going to rot if it's stored poorly. One bit of rubber that always is known to melt is the rubber feet on the charger for the PowerBook 500 series laptops. It ALWAYS melts. I've got one that hasn't done so yet though. Why? Because I believe that it was stored rubber-feet-up for however long it was in storage in the past. I left it for around a week, feet down, and it got slightly stuck to my desk. It's pretty clear to me that if I left it for a couple months like that, it would melt like all the others have. My assumption would be that it is chemically fragile enough that contact with surfaces will cause it to rapidly chemically decay.

Case in point, my PS/Note 425 (sold as the ThinkPad 350 as well) has the feet melted on one side but not the other. It was stored in a box sideways, with multiple other laptops in a garage for probably over 20 years. During which time, the front side feet were contacting something else in the box, while the upper ones weren't. That just about proves my theory for me.

This does get more tricky when you consider something like a 700-series ThinkPad that's completely coated in the stuff though. Environmental aspects clearly also play a role, which explains why you can still find non-rotted 701c laptops if you look hard enough. Don't you have a couple solidpro? I think I saw your post on the ThinkPad subreddit a while back. I assume you're the same solidpro anyway.

My rubber-coated T series laptops are all still ok, although my T30 has some strange discoloration on it. I can clean it off but it always comes back later on.
I used to think using anti-static bags, or nice new bubblewrap was good but in this case that is indeed 'storing poorly' because as you say anything touching, rots it.

That was me on reddit. I have about 4 x 701c machines - 2 have been sympathetically stripped of their rubber coating and are satin black now, which kinda looks factory if you didn't know better. The other two are not rotten at all. Most are though.

Does your T30 discolouration look a bit like grey mould?
Let me see if I can find a photo of mine. I’m not home at the moment.