BQN, 15 bytes^SBCS

{=𝕩?(≠⌈´𝕊¨)𝕩;0} 

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{=𝕩?(≠⌈´𝕊¨)𝕩;0} # Anonymous function taking the list as right argument 𝕩 =𝕩 # Rank of the argument; 0 for ints and 1 for lists   ? # If the rank is 1: 𝕊¨ 𝕩 # Recursive call on each element ≠⌈´ # Maximum reduction of the results starting with the length of 𝕩  ;0 # Otherwise: Return 0 

or questionable 12 bytes:

{(≠⌈´𝕊⎊0¨)𝕩} 

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This relies on exceeding the maximum call stack size, which means this only works if such a limit exists and is larger than the maximum depth of the ragged list.

BQN, 15 13 bytes^SBCS

-2 bytes thanks to Razetime!

{(≠⌈´𝕊⍟=¨)×𝕩} 

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{(≠⌈´𝕊⍟=¨)×𝕩}  # Anonymous function taking the list as right argument 𝕩 ×𝕩 # Sign of each number, converts all numbers to 1 =𝕩 ¨ # For each value in the list 𝕊⍟= # do a recursive call if it is a list (rank 1)   # and return integers unchanged (rank 0)  ≠⌈´ # Maximum reduction of the results starting with the length of 𝕩 

or questionable 12 bytes:

{(≠⌈´𝕊⎊0¨)𝕩} 

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This relies on exceeding the maximum call stack size, which means this only works if such a limit exists and is larger than the maximum depth of the ragged list.

BQN, 15 bytes^SBCS

{=𝕩?(≠⌈´𝕊¨)𝕩;0} 

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or questionable 12 bytes:

{(≠⌈´𝕊⎊0¨)𝕩} 

Run online!

This relies on exceeding the maximum call stack size, which means this only works if such a limit exists and is larger than the maximum depth of the ragged list.

BQN, 15 bytes^SBCS

{=𝕩?(≠⌈´𝕊¨)𝕩;0} 

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{=𝕩?(≠⌈´𝕊¨)𝕩;0} # Anonymous function taking the list as right argument 𝕩 =𝕩 # Rank of the argument; 0 for ints and 1 for lists ? # If the rank is 1: 𝕊¨ 𝕩 # Recursive call on each element ≠⌈´ # Maximum reduction of the results starting with the length of 𝕩 ;0 # Otherwise: Return 0 

or questionable 12 bytes:

{(≠⌈´𝕊⎊0¨)𝕩} 

Run online!

This relies on exceeding the maximum call stack size, which means this only works if such a limit exists and is larger than the maximum depth of the ragged list.