Many thanks to OP for opening
.. right up my street
I detect two main threads here: first, obsolescent but once 'mainstream' (usually Latin/Norman* or Anglo-Saxon) words, and, second, definitely 'regional' (often Celtic/Norse) ones. I like both, both being particularly useful for cryptic crosswords
. For the last, 'Chambers' (the best single-volume English dictionary with etymologies) distinguishes between 'obs
] and 'dial
*Greek-derived ones tend to be more scholarly, and tend to remain in use within their scholarly conexts, often scientific or analogously academic. Similarly, there are words of subcontinental origin, brought to UK from the Raj era, of which some have become mainstream (eg 'bungalow'), some widely understood but somewhat affected ('pukka' ?), and some now apparently obsolescent ('tiffin' ?), or now rare outside particular, often military, contexts ('Blighty' ?).
It is often apparent that these 'Raj' ones tend to be unknown to Americans, unsurprisingly. Analogously words of 'First Nation' origin may be known to them, but less common in UK. These, too, may be contemporary mainstream there ('caucus'), or obsolescent even there ('potlatch' 'mugwump').. It is also noticeable that the vocabulary may be different in US English (most commonly Algonquian, ie Eastern seaboard) and in Canadian English/French (various). Equally, there are many more words of Yiddish origin in American English than in UK, reflecting greater and more influential immigration; some are still mainstream ('schmuck'), some more community-based ('meshuggenah'), some obsolescent outside historical context ('shtetl').
In the very first category, I recall twice surprising US colleagues when in DoD by using perfectly sound past tenses of what to me were perfectly reasonable everyday words: 'forwent' on one occasion 'gainsaid' on another
, both of which were understood when my interlocutors thought about them for a moment.
In the second ('dialect') category, there are some splendid words in Newfoundland English; some from English/French/Portuguese, some from First Nation or Inuit words.
<mustn't go on too long, this is too much fun