You can’t use the IN clause like that. It compiles to a single string in your IN clause. But an IN clause needs separate values.
WHERE id_campo not in (@idcamposexcluidos)
compiles to
WHERE id_campo not in ('817,803,495')
but it should be
WHERE id_campo not in ('817','803','495')
To overcome this either use dynamic SQL or in MySQL you could use FIND_IN_SET:
SET @idcamposexcluidos="817,803,495";
...
WHERE FIND_IN_SET(id_campo, @idcamposexcluidos) = 0
but using a function like FIND_IN_SET() can not make use of indexes.