Does Shuckle really produce Rare Candies?
No, but Shuckle can turn a Berry into Berry Juice
A disassembly of Pokémon Crystal is available on GitHub, and nowhere in it is code for having Shuckle turning Berry Juice into a Rare Candy.
However, this rumor probably has its origin in the fact that Shuckle can turn a Berry into Berry Juice after the player has visited Goldenrod City. This section of code includes a function that runs after the player has visited Goldenrod City, then has a 1/16 chance of a Shuckle holding a Berry turning that into Berry Juice.
ConvertBerriesToBerryJuice:
; If we haven't been to Goldenrod City at least once,
; prevent Shuckle from turning held Berry into Berry Juice.
ld hl, wStatusFlags2
bit STATUSFLAGS2_REACHED_GOLDENROD_F, [hl]
ret z
call Random
cp 6 percent + 1 ; 1/16 chance
ret nc
ld hl, wPartyMons
ld a, [wPartyCount]
.partyMonLoop
push af
push hl
ld a, [hl]
cp SHUCKLE
jr nz, .loopMon
ld bc, MON_ITEM
add hl, bc
ld a, [hl]
cp BERRY
jr z, .convertToJuice
.loopMon
pop hl
ld bc, PARTYMON_STRUCT_LENGTH
add hl, bc
pop af
dec a
jr nz, .partyMonLoop
ret
.convertToJuice
ld a, BERRY_JUICE
ld [hl], a
pop hl
pop af
ret
No, Shuckle could never turn Berry Juice into a Rare Candy. That is just a rumor.
Here's a YouTuber going through the disassembled code to debunk it.