Does anyone know the smallest cog a Shimano Deerhead RD (1st gen Deore XT) can handle? According to disraeli gears, the max cog is 34t, and my current freewheel's smallest cog is 14t, but I'd like to run it on an 8 speed cassette with an 11t if possible. Mine is the superplate version, if that matters.

You may have to experiment. What is the max wrap for that derailleur? If your still trying to run the 34 but drop to a 11 with a wide Alpine type triple you may be maxing out the derailleur.

The max wrap according to disraeli is 40t. It's set up in a very wide TA triple now (Half-step plus granny, I think its 52-48-28ish). But I'm moving it to another frame to set up as a 44t single.

The manufacturers usually understate their RDs actual capacity by a significant margin. As a fairly inexperienced mechanic, I've never really considered "how small can you go" as question needing asking, only "how large."

FWIW, the Deore DX I have that's a couple generations after Deerhead works just fine on a 7-speed cassette I have that's 12-32. I bet it would go down to 11, but I don't have one to try, so I can't test that.

I guess the way I think about it is that the big cog is the limiting factor- the upper jockey has to be beneath the respective cogs. The distance is one of the primary factors to determine how well it shifts.

Deerhead was before Suntour's slant-parallelogram patent ran out, so it won't have optimal shifting on the smaller cogs; it just won't track the cassette that well. But it will work. Hyperglide cogs help.

I used that RDER & shifters from 12-28 7 speed, through 8 speed to a 12-23 9 speed on my 86 Rockhopper.
Although I had no need for an 11T small cog, I sincerely feel it wouldn't have been a problem.

Shifting actually got better as I added cogs, due to ergonomics of my thumb etc.
I probably hit my gear 95% of the time until I got N+1 with trigger shifters.
My friction shifting skills went down the tube.....