Yes, CAD is supported as a home currency in your QuickBooks Online (QBO) account, @EmBBo7. It's great to know that you're proceeding with caution before enabling multicurrency. It's essential to have clarity on this aspect to ensure accurate bookkeeping, especially since once it's is enabled, it cannot be turned off and the home currency cannot be altered. I want to ensure that I provide the necessary information to clarify this for you.
The message tooltip you've read is a generic default that appears as a precaution but doesn't necessarily reflect the most up-to-date information about available features. You can rest assured of proceeding with enabling it with CAD as the home currency as detailed in this QuickBooks article: Set up and use Multicurrency in QuickBooks Online.
To give a clearer picture, here's an image example of how a cheque would look like with CAD as your home currency and a payment made to a US company in USD after enabling it:

In this example, when you record an expense for a payment made to a US supplier in USD, QBO converts the transaction amount to your home currency (CAD) using the relevant exchange rate. Moreover, when you run financial reports, CAD will be the primary figure reported in financial statements.
To learn more about the multi-currency feature, I'm including this article that answers frequently asked questions about its use in QBO: Multi-currency FAQ.
I appreciate you for reaching out and being diligent about your accounting setup. If you have any further questions or need additional assistance, feel free to click on the Reply button. We are here to help you ensure your setup meets your business needs.