There are 99 spuhs in eBird that don't directly correspond to a genus, family, or order (or, in a couple cases, suborder) already reportable on iNat - should I add those (or all spuhs, to be consistent with the 52 slashes that comprise an entire genus) as allowed values for this observation field? How about all the eBird forms, too (including undescribed forms, phenotypically distinct forms that may or may not be taxonomically distinct, and a few groups of subspecies that don't correspond to Clements subspecies groups)?

Just added all the "spuhs" and forms, so there's now a total of 1326 allowed values. (I also added a note to the description on how to see the list of allowed values for people who might not be entirely familiar with observation fields - if you think that should be removed from the description, go ahead.)
Do you think it would be a good idea to put:
a. a link to the eBird Help article that explains what slashes/spuhs/etc. are, and/or
b. a link to download the eBird/Clements checklist
in the observation field description?

wow, let me know how that huge list works in this format. Also I have pasted urls into field descriptions, though they aren't clickable and may be a pain to get 'out'.

I added everything just to be consistent, but yeah, it's probably best to remove those - I'll do that now (and also remove all the slashes that correspond to a genus with only two species). (I'll list them in a comment after this one just to keep track of them in case any of them change in next year's Clements update)

(I've started making the list, but I have to go do something else - I'll be back to finish the list and revise the list of allowed values in a few hours)

Awesome. I think linking a journal post so it's copyable in one go is best. And it'll notify your followers that the new field exists. :)

I just added subfamilies to Furnariidae, so I removed woodcreeper sp. to the list of unused slashes and spuhs since it was no longer needed (unidentifiable woodcreepers can be reported as Dendrocolaptinae)

The list currently doesn't include any of the Clements checklist's polytypic subspecies groups; I think I'll make a separate observation field for them to make this one a little less cluttered, if that's ok.

(It would probably be ideal if these subspecies groups could eventually be integrated into iNat as actual iNat taxa [with the relevant subspecies included within them] rather than just as part of an observation field, especially since they're included in Clements [and not just the eBird version], which iNat follows. A couple more examples in addition to those you mentioned in the Google group are:
Junco hyemalis hyemalis [currently being used for any Slate-colored Junco];
J. h. oreganus [currently being used for any Oregon Junco];
Setophaga petechia erithachorides [currently being used for any Mangrove Warbler];
S. p. petechia [currently being used for any Golden Warbler];
Passerella iliaca iliaca [currently being used for any Red Fox Sparrow];
P. i. schistacea [currently being used for any Slate-colored Fox Sparrow];
P. i. unalaschcensis [currently being used for any Sooty Fox Sparrow]; and
P. i. megarhyncha and P. i. stephensi [both of which have the common name Thick-billed Fox Sparrow, and the former especially seems to be used for any Thick-billed Fox Sparrow].
It seems like it would be a shame if these ended up being swapped up to species level, losing thousands of associated narrower identifications, but at the same time, the current assignment to subspecies is inaccurate - being able to swap them to Clements's J. h. hyemalis/carolinensis, J. h. [oreganus Group], etc. would preserve the IDs with their original intent, plus it would save someone the effort of going through thousands of observations and manually adding the observation field to each one.)