Yes, crossposting is discouraged on the stackexchange network. For a detailed explanation why, check out "Is cross-posting a question on multiple Stack Exchange sites permitted if the question is on-topic for each site?" on the general stackexchange meta.
Do you have a fact-checking question and you wonder if it is more appropriate to post on skeptics or ...
When a comment thread gets gets about 20 replies long, I move the comments to chat, and I ultimately purge the entire comments section.
Occasionally, I'll leave the first few comments if they're obviously on topic, but comments like the following don't cut it.
The less attention paid to Donald Trump, the better. He has no more chance of being president ...
One possible idea I had (assuming community likes it and moderators don't hate it) is:
Create a canonical CW Q&A on main site,
Q: "What notable/useful questions that are on-topic on Politics.SE were posted across SE network"
A: List of links
Create a Meta Q&A, with the question being about same idea, and 1 link per answer
An answer with a link to ...