coinbaseaux is responsible for specifying what data goes into the root transaction's scriptSig, so it can't just be removed - you have to somehow replace its usage with coinbasetxn, whose value is a mock raw transaction with values that need to be filled in.
"Somehow", because I couldn't find much explanation about coinbaseaux.flags except for a vague stack exchange answer:
https://bitcoin.stackexchange.com/a/18901/112589Coinbaseaux flags has been empty and it's omission doesn't affect the scriptsig. The structure of the scriptsig, except for the block height being specified at the start won't affect the validity of it.
"coinbaseaux": {
"flags": ""
The omission of it *probably* won't result in any changes and probably being due to the fact that I don't have sufficient resources to mine a block for myself and attest to that statement. If you cannot review the code in its entirety and check if the changes would affect any other components, it would be better to just go and run 0.19.1 and save any potential headaches. As for the link in that post, the flag is 062f503253482f which decodes to /P2SH/, which was also removed in 2015 as it was previously used to monitor for P2SH signalling.