A 100 digit number cannot be encoded losslessly in D-Bus btw, nor in the far majority of IPC marshallings on this word.
Having done systems-level OS development since 25y or so I never felt the burning urge to send a 100 digit number over local IPC.
Not that 100 digit numbers aren't useful, even in IPC, but typically, that's a cryptography thing, and they generally use their own serializations anyway.
I know you know JSON is nominally only 53-bit safe, because JS numbers are doubles. But in practice I'd wager most JSON libraries can handle 64-bit integers.
I think the concern is that large numbers can in fact be encoded in JSON, but there is no guarantee that they will be decoded correctly by a receiver as the format is underspecified. So you have to cater for the ill defined common denominator.
Actually, not new. Earliest CVE I found was from 2017, which feels a decade later than it should be. I guess no one thought of pushing JSON over trusted interfaces, and probably for good reason.