Ask Samsung. None of their devices has 8.1 and made no mention of making 8.1 for any of their devices. Carriers don't touch code like most people think they do.
Software updates are released as they become available. While 8.1 is not yet available, when it is, it will appear here: https://www.verizonwireless.com/support/software-updates/
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It appears to be imminent, at least from Samsung:
Unfortunately, VZW customers most likely won't see it for six months to a year given Verizon's history in NOT providing timely updates for the Galaxy S9/S9+ (the S9/S9+ are still on Android® Security Patch Level 2018-02-01, released 10 March 2018). Other carriers have released much newer updates for the Galaxy S9/S9+, and VZW itself has released newer security patches/updates across the "current" Galaxy product line, including Galaxy S6, S6 edge, S7, S7 edge, S8, and S8+. Something is definitely wrong when my Corporate VZW Galaxy S6 has Android/Samsung security updates two months newer than my personal VZW Galaxy S9+!