@Jemison, Chad, Thanks for posting in Q&A. Based on my researching, currently, it is not possible to customize the conditional access error message yet. But your thought of security consideration is good. You can feedback in the following location to see if we can get this feature in the future.
https://feedback.azure.com/d365community/forum/22920db1-ad25-ec11-b6e6-000d3a4f0789
Thanks for your understanding.
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