declined - Using standard flags helps us prioritize problems and resolve them faster. Please familiarize yourself with the list of standard flags: see What is Flagging?.
Apart from the canonical MSO thread for not allowing cross-posting, here at DSSE, in contrast with several other SE sites, we explicitly request not to cross post in our on topic help page (emphasis in the original):
Don't post your questions on more than one site.
Unsurprisingly, the issue of cross-posting has been raised here at Meta a couple of times, and always the advice included to flag it for moderator attention; quoting from When is it okay to cross-post to on Data Science from Cross Validated?:
flag it for moderator attention
flag for moderator attention, with a custom flag, and explain your reasons in the textbox
The last advice is repeated verbatim in Identical question posted here and on Cross-Validated, where moreover, a site moderator weighs in, and advises:
it is OK to raise a moderator flag, since the moderators are here to handle the odd situation such as these.
Last but not least, a quick look at my flag history reveals that so far I have raised no less than thirty (30) similar custom flags for cross-posting, without a single one being ever declined for any reason.
Given the above, why my flag was declined?
In principle, I could understand a reason that I did not include an argument in the textbox regarding at which site the question is off-topic; but an advice for using standard flags instead is totally puzzling here.