Currently the option for not shuffling the outputs in Electrum ABC is only available when op_return data is specified. I'm wondering if I should make this option available for all send-to-many transactions, or if the spec should be adjusted to enforce the order of outputs only when there is an op_return.

There is clearly a use case for ordered outputs with op_return (SLP), but I'm not sure if there is any for non-opreturn transactions. So there is a tradeoff to be made between letting the user control the transaction he wants to make and not providing the user with options that can accidentally hurt his privacy.

I lean towards accepting this diff and changing Electrum ABC to enable ordered outputs in the case of URIs of if the user checks the option. I'm just wondering if anyone else has an objection.

wording suggestion

Please make sure that your parsers will be able to decode queries with several time the same parameter

That should work. With python, repeating the same parameter makes the parser put the results into a list.
There is some concatenation to do for the first address (the path of the URI)

>>> import urllib.parse
>>> u = urllib.parse.urlparse("ecash:prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07?amount=100&address=ecash:prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07&amount=200&address=ecash:prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07&amount=300")
>>> u
ParseResult(scheme='ecash', netloc='', path='prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07', params='', query='amount=100&address=ecash:prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07&amount=200&address=ecash:prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07&amount=300', fragment='')
>>> u.path
'prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07'
>>> pq = urllib.parse.parse_qs(u.query, keep_blank_values=True)
>>> pq
{'amount': ['100', '200', '300'], 'address': ['ecash:prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07', 'ecash:prfhcnyqnl5cgrnmlfmms675w93ld7mvvqd0y8lz07']}