My phone number was reported for spamming…?

You are not notified when someone has reported your messages a Spam. Messages is intended for communications between friends and family where they would not be reporting you as sending Spam. If you are sending messages to others, they certainly have the option to report your communications as Spam and you can expect your account to be disabled. It is your responsibility to follow the Terms of Use for your Apple Account and the sender does not make the determination if their message is Spam, it is the recipient that received the message.

In the terms of use, that specific violation is noted here:

• post or transmit spam, including but not limited to unsolicited or unauthorized advertising, promotional materials, or informational announcements;

No notice is required by Apple:

• Apple further reserves the right to modify, suspend, or discontinue the Services (or any part or Content thereof) at any time with or without notice to you, and Apple will not be liable to you or to any third party should it exercise such rights.
• If you fail, or Apple suspects that you have failed, to comply with any of the provisions of this Agreement, Apple may, without notice to you: (i) terminate this Agreement and/or your Apple Account, and you will remain liable for all amounts due under your Apple Account up to and including the date of termination; and/or (ii) terminate your license to the software; and/or (iii) preclude your access to the Services.
• Apple has adopted a policy to disable and/or terminate in appropriate circumstances the accounts of users who are found repeatedly to infringe or are repeatedly claimed to infringe the copyrights of others. As part of implementing such policy, Apple may in its sole discretion suspend, disable and/or terminate the accounts of users who have been identified as repeatedly engaging in infringing activities or for other related reasons.

Legal - Apple Media Services - Apple

The suspension will either be temporary or permanent depending on what Apple has determined. If it is temporary you may be allowed access to the service again and would need to contact Apple directly.

Contact Apple Support - Apple Support

Also, you should not be using iMessage in a Commercial/Business manner for communications. For example, a Realtor that sends out property interests to a list of users may certainly find that their account will be suspended for the violation if others are reporting those as Spam. For business use of messaging, you should be using a service that is intended for business/commercial use and that is not iMessage. Again, that Apple service is only intended for personal communications and friends and family are not reporting your messages as Spam.

Please update with any information Apple has provided for a resolution and if you haven't heard anything back in a week, I would contact them again. The Spam is certainly one reason why an account may be disabled, but not the only reason and since you are able to message some users, that may not be the reason at all.

Contact Apple Support - Apple Support

Of course as always some messages will not be sent as iMessage if the recipient has their iMessage turned off, not able to get an internet connection, or if they are using an Android phone. In those cases, the messages will default to the SMS/RCS protocol where you will see a green text bubble.

If you used a VPN while traveling overseas, I would recommend uninstalling that. A VPN can interfere with iMessage/FaceTime activations and just turning it off may not be enough. Apple does provide a Support Article about the services that may be affected when using a VPN and iMessage/FaceTime activations are one of those.

If your device has network connectivity issues, check for VPN and other third-party security software - Apple Support

Thanks for this information. I have been using iPhone for years and have never experienced this. I only message/contact friends and family, and do so very little might I add. So this was completely shocking. My service has been wonky since returning from Mexico on vacation. iMessage and FT access has gone in and out since then. I thought it was because I hadn’t updated to the newest IOS. I updated today and an error message popped up on my screen stating that iMessage had an error. I contacted Apple for help and they told me about the spam report. They stated they’d look into it, but nothing more.

as of right now, I am able to use iMessage with certain people in my contact list and with others it’s still only SMS option. This is all new and VERY weird to me.

Edit: Mac Jim ID found the articles I couldn't.