Telling someone what they should have said after the event always strikes me as particularly unhelpful.
Any advice that is just retaliation - ditto.
The 'did you mean to be so rude' snarking annoys me. The only time to use 'did you mean to be so rude' is when you have endlessly to deal with someone who says shitty things but pretends they didn't mean them or that you misunderstood. In my case it worked. I started mentioning it every time she did it and she had to stop. It was great. I'd had years of 'are you ok, you are not looking well' 'did ds2 dress himself this morning' 'DH has been away a long time - he must need the break' 'that dress is always so flattering on you when you are carrying extra weight' 'I admire how you don't bother the children with too many rules and manners - I couldn't'
When I started replying with 'gosh, that's not very kind' , 'I'm not sure how that sounded in your head but it's pretty rude' 'are you criticising the children - again?' 'Was that intentionally rude' it was all played out in social situations and it stopped her. She wanted a free pass to snipe but I made reference to it, asking her intention, every time.
She initially said 'oh sorry, I didn't mean it like that' a couple of times but it shone a light on it so she couldn't pretend anymore.
Her choice was then either to stop or to plough on with everyone aware of exactly what she was doing.
If you have never had to deal with someone who does endless low level sniping but acts like a victim when you eventually snap and say 'will you fucking stop' then it probably makes no sense. Equally if you are the sort of person who can say 'oh fuck off' straight away and don't need to keep a family social relationship going then sure, say that.
But if your wider family/social life is played out alongside someone who pretends to be lovely and smiles and says shitty things to dig at you, this might just help.
Of course it's fucking stupid thing to say to someone who is just being rude. But that was not why it was originally suggested.