I'm used to this and other networks and I know why asking to chat off-site too soon can be a red flag. But it can also be perfectly innocent, especially from a newbie who isn't familiar with this kind of site. For this reason I keep chat on site until I feel we're getting to know each other at least a bit - but I don't report just for offering to move to WhatsApp/wherever too early.
Should I? When a stranger who's only just joined messages asking my availability (I'm not a professional), it's a nuisance but not their fault so I explain. When they give their phone number in the second message, I generally say I don't share mine with strangers. But at that point would you report? If yes, what reason to select; it isn't (yet!) non-platonic and certainly isn't abusive language etc.
Obviously if early messages mention "finding the one" or hoping for a romantic relationship etc etc, that's non-platonic. Again, I explain, but I do then report. This is different, this is only a phone number shared.
Also, in profiles - if a phone number, or kik/Instagram/Telegram/Signal username is in a profile, is this reportable? Even in the extended FAQ I can't find this, but I from memory I thought it was warned against somewhere. I also tried editing my profile to see if that made guidelines visible, but no.
I don't want to get anyone into trouble, let alone banned, whose intentions are good. If they're under a misapprehension they aren't to blame! they may choose to leave once they're better informed, and realise it isn't what they were expecting, but reporting is a different matter.