In my case, it seems to be related to syspolicyd contacting Apple’s iCloud services.
I have then turned off various iCloud services, including Photo synching. After that, it seems that syspolicyd has stopped running constantly.
What’s left active in my case:
- iCloud Drive
- Safari
- Keychain
- Find My Mac
This suggests these were not the culprits.
I wouldn’t be surprised if it’s really the Photo sync service – I’ve had a similar issue on iOS two years ago where the iPhone apparently got stuck with some picture that it kept uploading endlessly, apparently due to some issue with it (and due to a bug in Apple’s software, of course).
However, I don’t see how that could relate to Steam and its files.
But perhaps you can, like I did, use Little Snitch to block all connections, and see if that helps. If it does, you may figure out what the problem is, too.
So, while this answer may not be particularly fitting to the question, others who search for problems with syspolicyd may find my answer helpful.